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

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

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.

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

Thank you for your response. I tried, didn't work. The only files I had under "sessionstore-backups" folder was a "recovery.baklz4" and a "recovery.jsonlz4", but they were dated after the incident and did not contain tabs that were open before the update. I saved them somewhere else anyway.

There was also a "crashes" folder with a file: "store.json.mozlz4", which shows a time shortly before the update. Not sure if it is related to the problem.

Could these tabs before the update saved anywhere else? Like I mentioned "Restore previous session" was checked at the Startup. Thank you.

I called for more help.

Did Firefox create an upgrade.jsonlz4 file in the sessionstore-backups folder as is done normally when you update ?

Backup the session files in the sessionstore-backups folder in the Firefox profile folder to make sure not to lose possible important session data. Do NOT close Firefox when Firefox is already running.

You will normally find these files in the sessionstore-backups folder:

• previous.jsonlz4 (cleanBackup: copy of sessionstore.jsonlz4 from previous session that was loaded successfully)
• recovery.jsonlz4 (latest version of sessionstore.jsonlz4 written during runtime)
• recovery.baklz4 (previous version of sessionstore.jsonlz4 written during runtime)
• upgrade.jsonlz4-<build_id> (backup created during an upgrade of Firefox)

You can copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.jsonlz4 to replace the current file (make sure to backup the current sessionstore.jsonlz4).

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Thank you. When I looked after this incident, the only files I had under "sessionstore-backups" folder was a "recovery.baklz4" and a "recovery.jsonlz4", but they were dated after the incident.

When I look now, I see an "upgrade.jsonlz4" file also, but that too is dated a day after the update that costed the previously open tabs.

I wonder if the "crashes" folder has anything to do with it.