Did you clear your history? The browsing history needs to be saved intact for tabs to be restored.

are you saying I should clear history now? or is that what caused this?

If you clear history, your tabs cannot be restored. Check the setting in Options > Privacy for clearing history. Is that box checked? If so, what settings do you have with "Settings"?

ah I think that might be what did the damage, the tracking checkbox "tell websites I don't want to be tracked" I had this checked for a month or so, though recently I unchecked it because I was trying to fix a behavior in tabs that was resetting my current tab location in the upper browser.

Wow such a small change and I didn't remember it or think it significant. Sorry for the high alert and thanks for helping me realize I caused the problem. Though I had no way of knowing that unchecking the trackblock option would erase all my current browser history. Well live and learn.

I have been using tab groups since they first appeared. I have 122 tabs in 13 groups. My General tabs is open (on startup) and I can see my tabs for that group. When I click Group Your Tabs I see all the groups but all are blank. Clicking any tab group (or hitting escape) will return me to my General tabs with a new tab showing the normal choices. This is as reported in the OP. BUT if I use the tab search for a tab that I know is in one of my blanked groups this tab is opened at the expected URL in position 1 of the of my visible tabs. This can repeated for any page that is in any of the blanked out groups.

I do not change my settings (Options) EVER. I have not done so probably since the spiralling version numbers started since FF 3 or whenever that was. The only exception was that one install about 6 months ago reset my home page which I had to revert. This is the ONLY time I have changed any settings for at least 2 years.

FF 19.02 W/XP SP 3

This cannot be a user problem as several people are reporting this behaviour.

Please let me know how to restore my tab groups, or let me know how to view the tab group settings (sqlite db?) so that I can manually recover these.

Andy.

Further to my last, in URL bar on partial match Switch To Tab will also bring the 'fully loaded' page to tab position 1 of the General group instead of loading the relevant group and then the required tab.

(Looking into sessionstore.js and various backups to see what has changed)

FYI, Andy.

it happened again to me and this time I know that Firefox history is not set to timed clearing of cache. The new information is when my windows (win7) updated it must have done something. Though this is not conclusive since I was using Firefox today before the event of erase to watch recorded TV episodes and left for lunch.

When I came back to resume all was lost.  I haven't added any add-ons to Firefox in many months.  I've removed untrusted or unreliable or just unknown add-ons back months ago as well (not recently as none are threatening the history) I'm still at a loss but it might be windows update somehow I'll have to look into exactly what it downloaded/installed last night.