Does it still happen in Troubleshoot Mode?

Go to about:support in the address bar, scroll down to the "Places Database" heading, click the "Verify Integrity" button and wait for it to complete. The output will indicate if there are errors with the database and if they were fixed.

You can do a malware scan, just to be sure.

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

It does occur in Troubleshoot mode. Places database:verify integrity showed no issues.

Did a couple malware scans with Eset nod32, everything checks out.

Web search suggested a Firefox reset, though I understand that wipes all custom code. I've been using the custom code for years without issues and its fairly complicated.