If you only have sessionstore.js and not the currently used sessionstore.jsonlz4 then you need to use other steps to make Firefox use sessionstore.js.

Firefox can still restore uncompressed sessionstore files, but to make this work you need to remove (rename) compressed files and rename the sessionstore-backups folder to ensure that Firefox can't fallback to compressed .jsonlz4 files.

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 a backup copy of the current sessionstore.jsonlz4).

Firefox 56+ releases compress the files in the sessionstore-backups folder with LZ4 (.jsonlz4 instead of .js), so it is no longer easy to inspect them.

See also:

• http://www.ghacks.net/2014/06/27/mozilla-launches-improved-session-restore-firefox-33/

By "two sessions open", do you mean two separate windows? Check the History menu. After using Restore Previous Session, Firefox will list closed windows that you can re-open from your previous session. Firefox remembers up to three closed windows, so hopefully it still remembers the one you're looking for.

Hi jscher2000. Thanks for your prompt response. Yes there were two windows open, one window had just a couple tabs open, while the other was my main session. It appeared that the main session closed first then the smaller session (another program forced them closed). When I re-opened firefox, and used the restore last session feature, only the one small session restored. I did go to the History tab, however, it just listed the websites visited by date, not per any sessions. So one website - for example - a streaming news service I use - that had been open, but I hadn't clicked in for a week, is not listed in recently visited tabs, even though it was open. I am not sure where to see a list of tabs from a recent session in the History- it is only by date visited that I see the history tab works. Is there another way to find these lost tabs besides scrolling through all the dates and websites that were not open during the session and picking out the tabs that were? Thanks

As cor-el described, Firefox 56 changed the names and formats of session history files. All the files are now compressed and have extensions like .jsonlz4 and .baklz4. The .js files are from before Firefox 56.

I don't know what you can recover at this point, but if you back up everything in the sessionstore-backups folder to a neutral location where Firefox won't update or delete it, you can examine what is in the files using a tool on my website:

https://www.jeffersonscher.com/res/scrounger.html

That should let you see whether the missing window is in any of the files, and then we could (hopefully) figure out how to convince Firefox to use that one.

Brad8098098098 said

I am not sure where to see a list of tabs from a recent session in the History- it is only by date visited that I see the history tab works.

It is most clear on the History menu that drops from the classic menu bar. That bar is normally hidden on Windows; you can tap the Alt key or F10 key to display it.

The other place is the History menu that is part of the graphical application menu: "3-bar" menu button > History

Ok J-scher2000. I am using the scrounger tool, I did find one of the json files has all the information I need. Thank you for that! I will be able to use the scrounger to see the websites I need.

Also - yes thank you I do see that History has some "restore" tabs, which I gather are to restore Firefox completely. I am still unclear as to where to look to see a session that was closed, as opposed to individual web pages listed - if that even exists?

Cor-El - I could not find any file called sessionstore.jsonlz4 in the main profile file - so I am not sure which file to rename and hesitant to try trial and error. I used the tool provided by j-scher2000, and can re-open the websites manually. The article to which you provided a link is still referring to the .js files, all of which is the process I did previously without result. If you know the name of the file or if I am missing something else please let me know! Thank you both for your help!

Firefox uses the sessionstore.jsonlz4 file in the profile folder to store session data. Firefox 56+ releases compress the sessionstore.js file and the files in the sessionstore-backups folder with LZ4 (.jsonlz4 instead of .js file extension).

The sessionstore.jsonlz4 file is only present when Firefox is closed. The sessionstore.jsonlz4 file is created from recovery.jsonlz4 when you close Firefox and is removed and copied to previous.jsonlz4 when you start Firefox to make is possible to restore the session at any time.

Cor-el - thank you yes, I did close Firefox and the sessionstore.jsonlz4 file appeared. I moved the backup file from the backups folder that had all the website information (that I discovered was the correct file by using jscher2000's "scrounger" tool on his website) into the profile file - renamed the old sessionstore file and renamed the backup file to sessionstore.jsonlz4. I reloaded firefox and hit restore previous session. However, only one, of the several websites from the session, was restored. What now?

Could you check the History menu for any available closed windows? And within each window, closed tabs? (The Scrounger list usually indicates which windows/tabs were open/closed in the file.)