Somewhere along FireFox 28>28 update or during an extension install or just restarting FF.. 50-100 tabs split to several tab groups went into single tab and all became "new tab" without its history. places.json is 30mb sessionstore.js is about 4mb so, most likely info is not lost and could be resotred. _closedWindows trick does not sute my case, because it's at the very end (not beginning) of sessionstore.js . safe mode did nothing as well. I've changed nothing so far, so I just open new URLs along those bunch of lost "new tabs" and after FF restarts _sometimes_ I lose those new tabs I've opened along as well. Sync was set, so maybe something useful is there (tabs, care less about history). Would be nive to save tab groups as well. I'm ready to go deep (hack, edit/wipe and convert) as long root cause and directions would be told to me.