Can you start Firefox in Diagnose Firefox issues using Troubleshoot Mode? After a fresh reboot, hold down the option key while starting Firefox and then click "Continue in Safe Mode.

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems for detailed information.

screenshot

This is a screenshot of what Firefox displays for me when I try to open it. :/ Holding down 'option' does nothing but change 'hide' to 'hide others'. :(

Did you double-click on the Firefox icon while holding down the Option key?

Yes, it's not doing anything differently.

Can you create a new Firefox profile? See https://support.mozilla.org/kb/Managing-profiles#w_starting-the-profile-manager

If that doesn't work either, you could drag Firefox out of the Applications folder and into the Trash, then download a new copy from mozilla.org/firefox/new and reinstall it. Ref:

• Uninstall Firefox from your computer
• Installing Firefox

Before uninstalling and reinstalling Firefox, you could also see if something here will help:

• http://kb.mozillazine.org/Browser_will_not_start_up
• Firefox won't start - find solutions
• http://kb.mozillazine.org/Firefox_hangs
• Firefox hangs or is not responding - How to fix