What do you have Firefox set to do when closed? see screenshot Lets start with you posting a screenshot of the files in your sessionstore-backups folder in your profile folder. In the detailed view showing date timestamp. see screenshots of mine What day did the issue start? Close Firefox to see the sessionstore.jsonlz4 in your profile folder.

• When closed, Firefox "will remember history".
• sessionstore.jsonlz4 can only be opened through Dolphin (as your screenshot suggested), and I'm using macOS. Tried the 'File Viewer' and this is how it looks like, not sure if this helps.

1st screenshot is good, but the 2nd is not. You need to go into your profile folder and then a folder called sessionstore-backups like my screenshot above. The 1st screenshot shows you how to get into your profile folder. The 2nd screenshot shows the files in the sessionstore-backups folder.

Let me get on my old mac from 2012 and see. The process should be the same.

This is from my old mac. see screenshots Post similar screenshot of the files.

Yes, I can see the file. I cannot open it though.

Files ending with lz4 are compressed and not human readable. You need to decompress them first if you want to read their contents.

However, could you first check the following:

(1) Is a window with the missing pinned tabs still available here:

• "3-bar" menu button > History > Recently Closed Windows
• (menu bar) History > Recently Closed Windows

(2) Is a non-visible window with the missing pinned tabs listed in Firefox View:

• Open Firefox View using either the file drawer icon at the left end of the tab bar or by typing/pasting about:firefoxview in the address bar and pressing Return to load it
• In the left column, click Open Tabs - Firefox should show a box for each open window on the right, occasionally a window is off screen or on a different desktop

If that doesn't help...

Back up the contents of your sessionstore-backups folder.

The kinds of files you may find among your sessionstore files are:

• 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

Sometimes you can tell from the last modification time which one will have your missing tabs, but not always.

Optional File Contents Preview

These compressed files are a pain to view, so I created a tool on my website to list out their contents. If you want to try that, you can drag and drop it onto the large box on the following page, then click the "Scrounge URLs" button:

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

If you don't get a list within 15 seconds, that probably means the script is caught in a loop. You may need to close the tab to avoid a tab crash and then try again in a new tab.

If you get a useful list, use the "Save List" button to archive it as a web page of clickable links for future reference, in case no other approach is successful.

Once you know which file you want to roll Firefox back to (bookmark any current tabs you don't want to lose), see: How to restore a browsing session from backup.

Thanks guys for the enthusiastic replies, I appreciate your willingness to help me! To summarize:

• jonzn4SUSE helped in discovering the possible cause of the error and located the lz4 file, and
• jscher2000 helped with trying to decode the lz4 file using Scrounge.

Unfortunately, decoding recovery.jsonlz4 leads me to a long text full of another code that it's really frustrating to pinpoint when the issue started. And to be honest, I give up. I'll move away from Firefox because it's happening more often lately.

Sincerely appreciate your help!

Unfortunately, decoding recovery.jsonlz4 leads me to a long text full of another code that it's really frustrating to pinpoint when the issue started.

Usually after you click "Scrounge URLs" you get a readable list. Sorry if that isn't working correctly.

As for a permanent solution, it's really not clear what is going wrong. You mentioned that the Undo Close Tab shortcut sometimes helps. Does it restore a closed window, or a bunch of tabs in the current window? (Command+Shift+N is more efficient for restoring a closed window.)

Undo Close Tab shortcut didn't do anything, it stays in a new blank page.

Undo Close Tab shortcut didn't do anything, it stays in a new blank page.

Undo Close Tab is Command+Shift+T, which I thought was sometimes useful. If nothing happens, perhaps Firefox did not restore your session at all. You could check whether either of these are available:

• "3-bar" menu button > History > Restore Previous Session
• (menu bar) History > Restore Previous Session

If they are missing or grayed, Firefox either restored your session already or it did not find anything to restore.

Until a solution is found, it's probably best to bookmark your regular tabs to a folder so you can open them more easily:

(1) Open the pages in individual tabs (2) Press Command+Shift+D to call up "Bookmark All Tabs" (3) Create a folder name and save

To open those tabs, you can right-click (or Ctrl+click) the folder and then "Open All Bookmarks".