That's not good. :(

It sounds like you've used this feature before successfully and you probably have Firefox set to "Show my windows and tabs from last time"

Have you tried restoring your session from the history menu?

Thanks, Yes I have FF configured to restore tabs from last time. I can live with the loss of the active tabs but the 'Tab Groups' I had saved are rather more precious. I have explored history but it was configured save no older than a couple of days so the important stuff isn't there.

I'm new to this and have run data recovery software to recover any sessionstore.* (.js/.bak) files other than those already in the single profile folder. No clue what I should be doing!

Where are urls from the 'Group My Tabs' saved?

"I have explored history but it was configured save no older than a couple of days so the important stuff isn't there."

That's the problem right there. I suspect you created those tab groups more than a couple of days ago so the urls dropped out of what you have saved in your history. This is why they tab groups disappeared. It's best to have your history set to the default which is "Remember History" - see this Restore previous session - Configure when Firefox shows your most recent tabs and windows article for more info.

Ouch, I had hoped that TAB GROUPS would be as inviolate as BOOKMARKS. The very idea that they can expire seriously affects their value.

In the meantime I have been attempting to extract any URL's I can from Recuva'd sessionstore.js'

Is there any other file that might carry tab/group history?

You browsing history is stored in places.sqlite in your profile folder.