The Restore menu is only for bookmarks, not open tabs. You might have a lot of bookmarks, too?

Did you already restore the session that is missing a window? Either:

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

If you are still running the same session where you lost the window, try one of these:

• Ctrl+Shift+N
• "3-bar" menu button > History > Recently Closed Windows
• "Library" toolbar button > History > Recently Closed Windows
• (menu bar) History > Recently Closed Windows

Any luck?

If that doesn't help...

Firefox creates numerous session history files, but because session history is only for the immediately previous session, it is unfortunately too easy to lose it. Could you start by making a backup of your existing session history files? Here's how:

(1) To 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. On that row, click the "Open Folder" button. This should launch Windows' File Explorer 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

Could you take a look at what you have and the date/time of the various files to see whether you think any of them would have the missing tabs?

Optional Preview

To preview the contents of a file, you can drag and drop it onto a tool on my site, then click Scrounge URLs:

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.

Restoration Technique

This involves tricking Firefox into restoring a particular file. Details available if you find a promising candidate.

Oops, didn't notice you were running Linux. Some details of the above will be different on your Firefox (for example, Open Folder => Open Directory).

Hallo @jscher2000: One of your suggestions was the solution --> a big "thank you !" to you.

I got everything back not having only closed session but also rebooted...

       3-bar menu button > History > Recently Closed Windows > Re-Open all windows.'

did it. I usually do back up my collection of tabs, but I had been a little lackadaisical as of late. All fixed now.