https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

Thanks, I figured out what was wrong. Firefox hadn't closed completely. I found a listing for Firefox under Processes even though it was closed under Applications. Once I cancelled the process, everything was OK again. I have to get into the habit of exiting Firefox fully.

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.

If you should be having problems with Firefox closing, please ask a new question.