Sorry to hear about this problem. When Firefox restores your previous session, that should include all the tab groups for both open tabs and the tabs you chose to "save and close."

I'm going to suggest a method of checking older session history files to see whether they contain the closed tab groups. Here are the steps for Windows, and probably they are similar for Linux:

(1) Open your profile folder...

You can open your current Firefox settings (AKA Firefox profile) folder using either

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

In the first table on the page, find the Profile Folder row and click the "Open Folder" or "Open Directory" button. This should launch your system file browser listing the contents of your current profile folder.

(2) Copy out session history files

In your profile folder, double-click into the sessionstore-backups folder. Save all files here to a safe location such as your Documents folder.

(3) What files did you find?

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 <= I don't know whether this exists when updates are installed by a package manager

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. Grouped tabs should appear under their window if they are in an open group, and in the separate saved groups section if they are a closed group.

OMG, thanks for telling this I am really very grateful I could find my saved groups in `upgrade.jsonlz4-20250726173436` which stored all my groups(the closed ones that were lost), I can manually pull them back in firefox so thanks a lot.