If none of the files in the sessionstore-backups folder have the missing tab groups the it is likely that they are lost.

Are you using Sync?

I don't know is pinned tabs and tab groups are included in Sync. If that is the case then you would see them on other connected devices.

You can backup the session files in the sessionstore-backups folder in the Firefox Profile Folder to make sure not to lose possible important session data. You can find these files in the sessionstore-backups folder:

previous.js (cleanBackup: copy of sessionstore.js from previous session that was loaded successfully)
recovery.js (latest version of the sessionstore written during runtime)
recovery.bak (previous version of the sessionstore written during runtime)
upgrade.js-<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.js to replace the current file (make a backup copy of the current sessionstore.js).

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

As I wrote before - recovery.bak and recovery.js are overwritten by FF, and previous.js was already bad - without my tabs (although that during the previous run all the tabs were ok). I was able to recover the most important tabs from history one by one, but some of them - can't.

The current Firefox 41.0.1 release isn't that old, so if you had updated to that version then you might still have an upgrade file that Firefox creates automatically when you update. That was the main reason for my above reply.

cor-el said

you might still have an upgrade file that Firefox creates automatically

Yea, now I remember, that I had update yesterday, but if you mean "upgrade.js" file at "sessionstore-backups" folder - it wasn't created, I haven't it.

Is there an "Old Firefox Data" folder on the desktop with such a file in the sessionstore-backups folder?

cor-el said

Is there an "Old Firefox Data" folder on the desktop with such a file in the sessionstore-backups folder?

I've been looking for all my drives, not only on the desktop. Nothing like the "Old Firefox Data" folder there.