UPDATE: Please see Common Access Cards on Mac (Updated)

(1) Install root certificates to your Keychain so that Safari trusts DOD sites. Follow these instructions.

(2) Buy a CAC reader. I like the SCM SCR3500.

(3) Update any drivers if necessary. For the SCM SCR3500 to work on Yosemite or later, you'll need to download the manufacturer's new drivers and install them. If you are running Mavericks or later, then installing drivers will require temporarily disabling System Integrity Protection. Instructions can be found here.

(4) Install software for CAC reader to work with Safari. I use CACKey.

(5) If you want to use your CAC with Firefox, you will need to add the CAC reader as a security device. Open Firefox and open the Preferences dialog, then move to the Advanced tab, then selected the Certificates sub-tab. Click on Security Devices.

The Security Device Manager will open. To add your CAC reader, you'll need to tell Firefox where the CAC library is. Click the Load button, highlighted below.

Once the dialog opens to load a new security module, enter a descriptive name (like CACKey) and then type in the location of the library. For CACKey, you'll type /Library/CACKey/libcackey.dylib and click OK.

Insert your CAC reader and then your card into the reader, and restart Firefox. You should now be able to use CAC log-in with various websites. You should also be able to access your work e-mail through Outlook Web Access.