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

Thanks Fred for your time and helpful reply. I will investigate your suggestion.

I encounter perhaps only a few times a year a Firefox crash where Tabs are not restored. But this is an issue I have noticed for years across many Firefox and Mac OS software upgrades. It seems unlikely to be corrupt local installation.

I want my current open Tabs state to be bullet proof and never lost.

It would be best if Firefox operates in such a way that open Tab state is guaranteed to be always recoverable. Even if Tab state was guaranteed recoverable a simple daily snapshot feature would still be useful. A more full-featured backup of open Tab state that could be browsed back in time like Mac Time Machine and Windows Task View would be better.

If you are using the same tabs, you could create a new folder in the Bookmarks Manager with those links.

In case of serious crashes there is always the possibility that files get corrupted and that you lose open tabs and windows although Firefox should still be able to recover by using recovery.jsonlz4 or recovery.baklz4.

After a crash you can 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 with Firefox closed.

• make sure to backup the current sessionstore.jsonlz4

I encounter perhaps only a few times a year a Firefox crash where Tabs are not restored. But this is an issue I have noticed for years across many Firefox and Mac OS software upgrades. It seems unlikely to be corrupt local installation. I want my current open Tabs state to be bullet proof and never lost.

The current design dating back to around Firefox 33 was intended to allow recovery even if two or three session history snapshot files became corrupted. Without specific scenarios to reproduce the loss of files, it would be hard to know how to improve it.

I think the most common reason I've seen for file damage is a power cut, so a battery to keep the computer running, such as in a laptop or an uninterruptible power supply, could be a tool to help protect against that scenario.

Thanks all for the helpful information. I have encountered articles on how to restore Tabs using the session recovery files. I actually tried that before this post and it didnt work. Probably I did it wrong.

I will again test restoring Tabs with the session recovery files as I see it is essentially the answer to this post.

An improvement I can see for end user experience is to make Tab session recovery in the event of a Firefox crash more user friendly than the current situation of having to search for articles on Tab restore and do some Firefox profile file manipulation.

It would be great if Firefox after a crash would prompt users to restore Tabs from the session recovery files and do the file manipulation automatically.

The important point is bullet proof Tab restore which means accessible and easy to all users. If there is a way to do it but its not widely known or accessible to most users - its useless and may as well not exist.