1. In Firefox, select Tools > Options.
     The Options dialog box appears.
  2. Click the Advanced icon.
  3. Select the Encryption tab, then click View Certificates.
     The Certificate Manager dialog box appears.
  4. Select the Authorities tab, then click Import.
  5. Browse to select the certificate file, then click Open.
  6. In the Downloading Certificate dialog box, select the Trust this CA to identify web sites check box. Click OK.
  7. Click OK twice to close the Certificate Manager and Options dialog boxes.
  8. Restart Firefox.

I have a WatchGuard XTM appliance. This is a security firewall. It has the capability of unencrypting https traffic for inspection and encrypting it again. To have it work for my Mac I had to import a certificate from the appliance and add it to Keychain. Safari now works. Next question - How do I add this cert to Firefox (3.6.8) for Mac. It's easy for PC. Please see below: Import a PEM Format Certificate with Mozilla Firefox 3.x Mozilla Firefox uses a private certificate store instead of the operating system certificate store. If clients on your network use the Firefox browser, you must import the certificate into the Firefox certificate store even if you have already imported the certificate on the host operating system. When you have more than one Firebox or XTM device that uses a self-signed certificate for HTTPS content inspection, clients on your network must import a copy of each Firebox or XTM device certificate. However, the default self-signed Firebox or XTM device certificates use the same name, and Mozilla Firefox only recognizes the first certificate you import when more than one certificate has the same name. We recommend that you replace the default self-signed certificates with a certificate signed by a different CA, and then distribute those CA certificates to each client. 1. In Firefox, select Tools > Options. The Options dialog box appears. 2. Click the Advanced icon. 3. Select the Encryption tab, then click View Certificates. The Certificate Manager dialog box appears. 4. Select the Authorities tab, then click Import. 5. Browse to select the certificate file, then click Open. 6. In the Downloading Certificate dialog box, select the Trust this CA to identify web sites check box. Click OK. 7. Click OK twice to close the Certificate Manager and Options dialog boxes. 8. Restart Firefox.