Upgrading to macOS seems me the easiest solution in this case.

Maybe the new functionalities of the current version aren't supported by Yosemite.

Firefox should work on OS X 10.10 (10.9+).

Try to start Firefox in Safe Mode to see whether that works.

• Windows: hold down the Shift key while starting Firefox
• Mac: hold down the Options key while starting Firefox

You can create a new profile to test if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

mattcamp said

Upgrading to macOS seems me the easiest solution in this case. Maybe the new functionalities of the current version aren't supported by Yosemite.

Thanks for your suggestions. Changing my Firefox profile sounds like a good possibility! However, I spoke to apple before I saw your comment, and at their suggestion, I updated to Sierra. Unfortunately, when the update was completed, I discovered that all my documents were missing other than those on my desktop. I've spent the past two hours with a supervisor, and we've tried reinstalling Sierra. It didn't work. She's taken a break and will call me back at 5:15. I'd appreciate any new comments that might help. Thank you so much!!!