Hi, hold down the Shift key when you start Firefox to start in Safe Mode. More - Diagnose Firefox issues using Troubleshoot Mode.

If it works in Safe Mode, disable any theme or extensions you have - 3-bar menu > Add-ons > Themes/Extensions and restart Firefox; then if it's OK, re-enable them one by one until you find the culprit. You may need to restart Firefox after enabling some extensions.

If that doesn't help, open the 3-bar menu > Help > Troubleshooting Information, then near the bottom look for Places Database and click on Verify Integrity, then restart Firefox - Use the Troubleshooting Information page to help fix Firefox issues. If that doesn't help, you may have a corrupt places.sqlite file in your Profile. For that, and other things to check, please see - Can't add, change or save bookmarks - How to fix.

If your question is resolved by this or another answer, please take a minute to let us know. Thank you!

After the above,

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.

Looks like a refresh fixed the problem.

I did try starting in safe mode and Verify Integrity. Safe mode didn't change anything and it failed the Verify Integrity.

Thank you very much.

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