If the browser thinks it crashed or did not close properly, it will automatically try to restore the last session.

Make sure you close the browser using; Windows: Menu > File > Exit Mac: Menu > Firefox > Quit Firefox Linux: Menu > File > Quit

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 the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show 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.

There seems to be no difference between closing Firefox in the usual way (click on top right 'X') and 'menu', 'exit' or Ctrl+Shift+Q. Deleting the 'sessionstore' files and directory does fix the problem - once - but it comes back.

I played around a bit and it seems that if I add a new tab and close Firefox, then re-start it, it does a session-restore correctly. But if then repeat the close and re-start I get the 'sorry, can't restore' message. Another close and re-start gets things back to where they should be with just my normal 3 start-up tabs. Is Firefox supposed to create sessionstore files even if the box in settings, general, startup is NOT checked?

I called for more help.

I hoped that version 97 (or M$ Tuesday) might have fixed the problem, but no such luck! The bug is annoyingly intermittent, but at present I'm getting the 'Sorry, can't restore' message about half of the time. There seems to be a definite problem with Firefox not handling the 'restore session' properly. In my case the option is definitley NOT ticked, yet FF tries to do it anyway, but most of the time it can't get it right! Once I select 'New Session', things get back to normal.

Use these links to create a new profile. Use this new profile as is. Is the problem still there?

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

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

Thanks for the quick reply Fred. I'm not exactly sure what you're recommending. I think it amounts to the 'Refresh' Firefox option. I did try that once, but it was a disaster and I ended up doing a full system restore from backup. If I have to sacrifice my passwords, bookmarks & cookies, I'll just live with the annoyance and hope that a future update fixes the bug. (OK, the cookies could do with a cleanup, but re-establishing the important ones is a bit of a drag.)

I suspect that what you really expect me to do is the equivalent of the Refresh, but then, if the problem goes away, to gradually copy my essential files back into the new profile, as suggested in one of the above links. But the list of files in the profile seems quite long!

I think it amounts to the 'Refresh' Firefox

Yes, and no. If the problem is gone with the new profile, we can Copy parts of the original profile and recover your data.

I created a new profile, set up one home page (Wikipedia), then opened a second tab with another site and quit (using the menu). Start up still gives me the 'sorry, can't restore' message. So the option restore flag is definitely being ignored. (It does seem to be OFF by default - I did check.) The profile manager allowed me to get back to normal quite easily, so FF works as advertised in that respect.

P.S. One additional bit of information: FF almost acts as if it wants me to choose whether to restore or start new. Normally I just click on start new, but if I click on 'try again' it does restore successfully (most of the time).

Update: Partial diagnosis but no solution! The problem appears to be associated with the Avast AV, since if I turn Avast off temporarily, the Firefox problem goes away. The bad news is that I don't seem to be able to fix the problem by just making an 'Exception' for the FF profiles folder. Obviously, I'm not willing to run this computer with my AV permanently turned off! (OK, I know that M$'s Defender will leap into the breach, but I don't trust M$ that far!) Is the FF restore/new flag stored in the FF profile?

Final update (I hope): The latest update - 97.0.1 seems to have fixed the problem. Thanks to the smart programmer responsible.

Sorry for the late reply. Been in Hospital.

Glad the problem is solved.