You may have corrupt sessionstore [v56] sessionstore.jsonlz4 file(s). Delete all sessionstore* files and the sessionstore-backups folder.

Don't delete the files if you need to rescue any data from them, just move them out of the profile folder to some location where Firefox doesn't look for them. You can try to read out their contents using this tool: https://www.jeffersonscher.com/res/scrounger.html

Type about:support<enter> in the address box.

Under Troubleshooting Information on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show or Open Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder

Linux: Under the page logo on the left side, you will see Application Basics. Under this find Profile Directory. To its right press the button Open Directory.

Locate the above file. Then rename or delete it. Restart Firefox.

Thanks. This is perhaps a little above my pay grade, but failing all else, I'll get to it. Finding the time is an issue. But on that, on exploration, it appears v97 was installed on or about precisely the time the tabs vanished. Any possible connection? Can it be reversed? cheers

That I can't say. But v97 has a big problem;

Could this be relevant? A couple of other users identified Webroot (security software) as blocking 64-bit Firefox 97 -- released today -- from running properly:

• https://support.mozilla.org/questions/1367248
• https://support.mozilla.org/questions/1367301

If you are still running an earlier release of Firefox, this would not be your issue.

After consulting with WebRoot, they advised this was a known issue. The fix was to remove and reinstalled the WebRoot application.