If you purchased your Mac with OS Catalina (10.15.x) already installed, you can skip the uninstall part above and follow the instructions below.

6 'high level' steps needed, follow down the page to make this a painless systematic process

1.Is your CAC reader 'Mac friendly'?
2.Can your Mac 'see' the reader?
3.Verify which version of Mac OS you have
4.Figure out which CAC (ID card) you have
5.Install the DoD certificates
5a.Additional DoD certificate installation instructions for Firefox users
6.Decide which CAC enabler you want to use (except for 10.12-.15)
Step 1: Is your CAC reader Mac friendly?
Visit the USB Readers page to verify the CAC reader you have is Mac friendly.
Visit the USB-C Readers page to verify the CAC reader you have is Mac friendly.
'Some, not all' CAC readers may need to have a driver installed to make it work.
NOTE: Readers such as: SCR-331 & SCR-3500A may need a firmware update (NO OTHER Readers need firmware updates).
Information about these specific readers are in Step 2
Step 2: Can your Mac 'see' the reader?

Plug the CAC reader into an open USB port before proceeding, give it a few moments to install

Step 2a: Click the Apple Icon in the upper left corner of the desktop, select 'About This Mac'
Step 2b: Click 'System Report..' (button)
Step 2c: Verify the CAC reader shows in Hardware, USB, under USB Device Tree. Different readers will show differently, most readers have no problem in this step. See Step 2c1 for specific reader issues.
Step 2c1: Verify firmware version on your SCR-331 or GSR-202, 202V, 203 CAC, or SCR-3500a reader. If you have a reader other than these 5, Proceed directly to step 3
Step 2c1a-SCR-331 reader
If your reader does not look like this, go to the next step.
In the 'Hardware' drop down, click 'USB.' On the right side of the screen under 'USB Device Tree' the window will display all hardware plugged into the USB ports on your Mac. Look for “SCRx31 USB Smart Card Reader.” If the Smart Card reader is present, look at 'Version' in the lower right corner of this box: If you have a number below 5.25, you need to update your firmware to 5.25. If you are already at 5.25, your reader is installed on your system, and no further hardware changes are required. You can now Quit System Profiler and continue to Step 3.
Step 2c1b-SCR-3500A reader
If you have the SCR3500A P/N:905430-1 CAC reader,you may need to install this driver, as the one that installs automatically will not work on most Macs. Hold the control key [on your keyboard] when clicking the .pkg file [with your mouse], select [the word] Open
Step 3: Verify which version of MacOS do you have?
(You need to know this information for step 6)
Step 3a: Click the Apple Icon in the upper left corner of your desktop and select 'About This Mac'
Step 3b: Look below Mac OS X for: Example: Version 10.X.X.
Step 4: Figure out which CAC (ID Card) you have
(You need to know this information for step 6)
Look at the top back of your ID card for these card types. If you have any version other than the six shown below, you need to visit an ID card office and have it replaced. All CACs [other than these six] were supposed to be replaced prior to 1 October 2012.
Find out how to flip card over video
Step 5: Install the DoD certificates (for Safari and Chrome Users)
Go to Keychain Access

Click: Go (top of screen), Utilities, double click Keychain Access.app

(You can also type: keychain access using Spotlight (this is my preferred method))

Select login (under Keychains),and All Items (under Category).

Download the 5 files via links below (you may need to <ctrl> click, select Download Linked File As.. on each link) Save to your downloads folder
Please know.. IF You have any DoD certificates already located in your keychain access, you will need to delete them prior to running the AllCerts.p7b file below.
https://militarycac.com/maccerts/AllCerts.p7b,
https://militarycac.com/maccerts/RootCert2.cer,
Free software to open winrar files. https://militarycac.com/maccerts/RootCert3.cer,
https://militarycac.com/maccerts/RootCert4.cer, and
Double click each of the files to install certificates into the login section of keychain
Select the Kind column, verify the arrow is pointing up, scroll down to certificate, look for all of the following certificates:
DOD EMAIL CA-33 through DOD EMAIL CA-34,
DOD EMAIL CA-39 through DOD EMAIL CA-44,
DOD EMAIL CA-49 through DOD EMAIL CA-52,
DOD EMAIL CA-59,
DOD ID CA-33 through DOD ID CA-34,
DOD ID CA-39 through DOD ID CA-44,
DOD ID CA-49 through DOD ID CA-52,
DOD ID CA-59
DOD ID SW CA-35 through DOD ID SW CA-38,
DOD ID SW CA-45 through DOD ID SW CA-48,
DoD Root CA 2 through DoD Root CA 5,
DOD SW CA-53 through DOD SW CA-58, and
DOD SW CA-60 through DOD SW CA-61
NOTE: If you are missing any of the above certificates, you have 2 choices,
1. Delete all of them, and re-run the 5 files above, or
2. Download the allcerts.zip file and install each of the certificates you are missing individually.
Errors:
Error 100001 Solution
Error 100013 Solution
You may notice some of the certificates will have a red circle with a white X . This means your computer does not trust those certificates
You need to manually trust the DoD Root CA 2, 3, 4, & 5 certificates
Double click each of the DoD Root CA certificates, select the triangle next to Trust, in the When using this certificate: select Always Trust, repeat until all 4 do not have the red circle with a white X.
You may be prompted to enter computer password when you close the window Free graphic art software for mac.
Once you select Always Trust, your icon will have a light blue circle with a white + on it.
The 'bad certs' that have caused problems for Windows users may show up in the keychain access section on some Macs. These need to be deleted / moved to trash.

How Do I Load Firefox

The DoD Root CA 2 & 3 you are removing has a light blue frame, leave the yellow frame version. The icons may or may not have a red circle with the white x
or DoD Interoperability Root CA 1 or CA 2 certificate
DoD Root CA 2 or 3 (light blue frame ONLY) certificate
or Federal Bridge CA 2016 or 2013 certificate
or Federal Common Policy CAcertificate
or or SHA-1 Federal Root CA G2 certificate
or US DoD CCEB Interoperability Root CA 1 certificate
If you have tried accessing CAC enabled sites prior to following these instructions, please go through this page before proceeding
Clearing the keychain (opens a new page)
Please come back to this page to continue installation instructions.
Step 5a: DoD certificate installation instructions for Firefox users
NOTE: Firefox will not work on Catalina (10.15.x), or last 4 versions of Mac OS if using the native Apple smartcard ability
Download AllCerts.zip, [remember where you save it].
double click the allcerts.zip file (it'll automatically extract into a new folder)
Option 1 to install the certificates (semi automated):
From inside the AllCerts extracted folder, select all of the certificates
<control> click (or Right click) the selected certificates, select Open With, Other..
In the Enable (selection box), change to All Applications
Select Firefox, then Open
You will see several dozen browser tabs open up, let it open as many as it wants.
You will eventually start seeing either of the 2 messages shown next

How Can I Download Firefox

If the certificate is not already in Firefox, a window will pop up stating 'You have been asked to trust a new Certificate Authority (CA).'
Check all three boxes to allow the certificate to: identify websites, identify email users, and identify software developers
or
'Alert This certificate is already installed as a certificate authority.' Click OK
Once you've added all of the certificates..
• Click Firefox (word) (upper left of your screen)
• Preferences
• Advanced (tab)
• Press Network under the Advanced Tab
• In the Cached Web Content section, click Clear Now (button).
• Quit Firefox and restart it
Option 2 to install the certificates (very tedious manual):
How do i get firefox on my macbook air
Click Firefox (word) (upper left of your screen)
Preferences
Advanced (tab on left side of screen)
Certificates (tab)
View Certificates (button)
Authorities (tab)
Import (button)
Browse to the DoD certificates (AllCerts) extracted folder you downloaded and extracted above.
Note: You have to do this step for every single certificate
Note2: If the certificate is already in Firefox, a window will pop up stating: 'Alert This certificate is already installed as a certificate authority (CA).' Click OK
Note3: If the certificate is not already in Firefox, a window will pop up stating 'You have been asked to trust a new Certificate Authority (CA).'
Check all three boxes to allow the certificate to: identify websites, identify email users, and identify software developers
Once you've added all of the certificates..
• Click Firefox (word) (upper left of your screen)
• Preferences
• Advanced (tab)
• Press Network under the Advanced Tab
• In the Cached Web Content section, click Clear Now (button).
• Quit Firefox and restart it
Step 6: Decide which CAC enabler you can / want to use
Only for Mac El Capitan (10.11.x or older)
After installing the CAC enabler, restart the computer and go to a CAC enabled website
NOTE: Mac OS Sierra (10.12.x), High Sierra (10.13.x), Mojave (10.14.x) or Catalina (10.15.x) computers no longer need a CAC Enabler.
Try to access the CAC enabled site you need to access now
Mac support provided by: Michael Danberry
This article describes how to uninstall the Firefox application for supported operating systems. If you are planning to uninstall and reinstall Firefox to fix problems you may be having, you should first read Troubleshoot and diagnose Firefox problems.

Table of Contents

To uninstall Firefox, follow these steps:
  1. Close Firefox (if Firefox is open): Click the Firefox menu and select Exit.Click the Firefox menu at the top of the screen and select Quit Firefox.Click the Firefox menu and select Quit.
  2. Open the Applications folder in Finder.
  3. Drag the Firefox application to the Trash.
Your uninstall of Firefox is finished.

How Do I Get Firefox On My Mac

If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux for details.If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory to uninstall Firefox.
Uninstalling Firefox does not remove your user profile, which includes personal information such as bookmarks, passwords and cookies. If you also want to remove this information, you should remove the folder that contains your Firefox profile, which is stored in a separate location from the Firefox program.
  1. If you want to preserve your bookmarks, passwords, cookies, and other user data and settings, see Back up and restore information in Firefox profiles.
  2. Delete the firefox folder that contains your profile folder and the profiles.ini fileDelete the Firefox folder that contains the Profiles folder and the profiles.ini file - see How do I find my profile? for the location.
To begin the process of uninstalling Firefox, follow these steps:
  1. Close Firefox (if Firefox is open): Click the Firefox menu and select Exit.Click the Firefox menu at the top of the screen and select Quit Firefox.Click the Firefox menu and select Quit.
  2. Click the Windows button and select Control Panel.
  3. In the Control Panel window, click on Add or Remove Programs. The Add or Remove Programs window will open.
  4. From the list of currently installed programs, select Mozilla Firefox.
  5. To begin the uninstall, click the button to the right of Mozilla Firefox.
    If the Uninstall Wizard does not run, you can start it manually by running helper.exe, which is located by default in C:Program FilesMozilla Firefoxuninstallhelper.exe.
  6. In the Mozilla Firefox Uninstall Wizard that opens, click .
  7. Click .
  8. Click .
  9. To remove other files and folders that may not be removed by the uninstall, you must manually delete the Firefox installation folder, which is located by default in C:Program FilesMozilla Firefox.
  1. Close Firefox (if Firefox is open): Click the Firefox menu and select Exit.Click the Firefox menu at the top of the screen and select Quit Firefox.Click the Firefox menu and select Quit.
  2. Click the Windows Start button or press the Windows key .
  3. In the Start Menu, click Control Panel.
  4. In the Control Panel window, click the Uninstall a program link under the Programs section.
  5. From the list of currently installed programs, select Mozilla Firefox.
  6. To begin the uninstall, click the button at the top of the list.
    If the Mozilla Firefox Uninstall window does not open, you can start it manually by running helper.exe, which is normally found in one of these locations by default:
    C:Program FilesMozilla Firefoxuninstallhelper.exe
    C:Program Files (x86)Mozilla Firefoxuninstallhelper.exe
  7. The Mozilla Firefox Uninstall window will open.
  1. Close Firefox (if Firefox is open): Click the Firefox menu and select Exit.Click the Firefox menu at the top of the screen and select Quit Firefox.Click the Firefox menu and select Quit.
  2. From the Start Screen, click the Desktop tile. The Desktop view will open.
  3. From the Desktop, hover in the lower right-hand corner to access the Charms.
  4. Select Control Panel from the Settings charm. The Control Panel window will open.

  5. In the Control Panel window, click the Uninstall a program link under the Programs section.
  6. From the list of currently installed programs, select Mozilla Firefox.
  7. To begin the uninstall, click the button at the top of the list.
    If the Mozilla Firefox Uninstall window does not open, you can start it manually by running helper.exe, which is normally found in one of these locations by default:
    C:Program FilesMozilla Firefoxuninstallhelper.exe
    C:Program Files (x86)Mozilla Firefoxuninstallhelper.exe
  8. The Mozilla Firefox Uninstall window will open.
  1. Close Firefox (if Firefox is open): Click the Firefox menu and select Exit.Click the Firefox menu at the top of the screen and select Quit Firefox.Click the Firefox menu and select Quit.
  2. Click the Windows Start button or press the Windows key .
  3. In the Start menu, select Settings.
  4. In Settings, select System and then Apps & features.
  5. From the list of currently installed programs, select Mozilla Firefox.
  6. To begin the uninstall, click .
    If the Mozilla Firefox Uninstall window does not open, you can start it manually by running helper.exe, which is normally found in one of these locations by default:
    C:Program FilesMozilla Firefoxuninstallhelper.exe
    C:Program Files (x86)Mozilla Firefoxuninstallhelper.exe
  7. The Mozilla Firefox Uninstall window will open.
Note: Instead of uninstalling Firefox, you can try the Refresh Firefox option included in the uninstaller. This can fix many issues by restoring default settings and removing add-ons.
To continue uninstalling Firefox, follow these steps:
  1. Click in the Mozilla Firefox Uninstall window.
  2. Click .
  3. Click to complete the uninstall.
  4. To remove other files and folders that may not be removed by the uninstall, you must manually delete the Firefox program folder from the location shown in the Uninstall Mozilla Firefox window (normally by default, C:Program FilesMozilla Firefox or C:Program Files (x86)Mozilla Firefox).
The uninstaller does not remove any user data such as history or bookmarks. If you wish to completely remove this data, you must manually delete the Firefox folder containing your user profile:
  1. Click the Windows button and select Run….
  2. Type in %APPDATA% in the Run prompt then click .
  3. Open the Mozilla folder.
  4. Delete the Firefox folder.
  1. Click the Start button or press the Windows key to open the Start Menu.
  2. Type %APPDATA% in the search field and press enter to open the hidden Roaming folder.
  3. Open the Mozilla folder.
  4. Delete the Firefox folder.
  1. From the Start Screen, click the Desktop tile. The Desktop view will open.
  2. From the Desktop, hover in the lower right-hand corner to access the Charms.
  3. Select the Search charm. The Search sidebar will open.
  4. Type %APPDATA% in the search field and press enter to open the hidden Roaming folder.
  5. Open the Mozilla folder.
  6. Delete the Firefox folder.
  1. Click the Start button or press the Windows key to open the Start Menu.
  2. Type %APPDATA% (when you start typing, a Windows search will start) and press enter to open the hidden Roaming folder.
  3. Open the Mozilla folder.
  4. Delete the Firefox folder.
Alternatively, you can press the Windows key +R to open a Run dialog, type in %APPDATA%Mozilla and click to open the Mozilla folder, then delete the Firefox folder.

Based on information from Uninstalling Firefox (mozillaZine KB)