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.

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

FredMcD said

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

I have old file "previous.js". Can i use it to restore my tabs in Firefox? Not just read.

I called for more help.

If "Restore Previous Session" is grayed, check these for potentially recoverable windows:

• "Library" toolbar button > History > Recently Closed Windows (and within each restored window, Recently Closed Tabs)
• (menu bar) History > Recently Closed Windows (and within each restored window, Recently Closed Tabs)

DendyGamer said

FredMcD said

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

I have old file "previous.js". Can i use it to restore my tabs in Firefox? Not just read.

The previous.js file name hasn't been used since Firefox 55, so that is likely to be an old session. Did you use the Scrounger to confirm that it is what you want to restore? Do you have any more recent versions of these files in the sessionstore-backups folder:

• recovery.jsonlz4: the windows and tabs in your currently live Firefox session (or, if Firefox crashed at the last shutdown and is still closed, your last session)
• recovery.baklz4: a backup copy of recovery.jsonlz4
• previous.jsonlz4: the windows and tabs in your last Firefox session
• upgrade.jsonlz4-build_id: the windows and tabs in the Firefox session that was live at the time of your last update

The general strategy for restoring an old session is:

(A) Hide the sessionstore.jsonlz4 file and the sessionstore-backups folder from your latest session (while Firefox is closed)

(B) Set up the old file in the profile folder as sessionstore.jsonlz4 or, for older .js files, sessionstore.js, then start Firefox and Restore Previous Session

In more detail:

(1) From inside Firefox, open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button. This should launch a Windows Explorer window listing the various files and folders in your profile.

(2) Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button Exit
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup -- these changes should occur:

• recovery.jsonlz4 is moved from the sessionstore-backups folder to the main level of the profile folder and renamed to sessionstore.jsonlz4
• recovery.baklz4 is deleted from sessionstore-backups

Once Firefox has stopped updating things:

(3) While viewing the main level of the profile folder, right-click > Rename the sessionstore.jsonlz4 file in the profile folder to sessionstoreOLD.jsonlz4

If you see an older sessionstore.js or sessionstore.bak file, move it somewhere else or delete it.

(4) Copy the old file you want to use and paste it into the current profile folder (please don't move the original in case this takes multiple tries)

(5) right-click > Rename the old file to sessionstore.jsonlz4

If the old file does not already have "lz4" somewhere in its file extension, it may be from the Firefox 55 era. In that case, rename that file to sessionstore.js so Firefox knows it is not compressed

(6) right-click > Rename the sessionstore-backups folder to sessionstore-backupsOLD so Firefox doesn't try to use those files (it will create a new one)

(7) Start Firefox and it should read the swapped in sessionstore file and restore whatever was in the substitute file; if you do not have Firefox set to restore your previous windows and tabs automatically, use Restore Previous Session.

Success?