I've had a similar problem with FireFox version 30 as found in the following question: https://support.mozilla.org/en-US/questions/997956?esab=a&as=aaq I lost all my tabs including those in two groups (maybe 100 tabs) that I'd been keeping for a couple of years and in some cases much longer. Very occasionally I'll get the "oops this is embarrassing" message from FF and when I click on the Restore Previous Session button they reappear. What happened this time is that when booting my computer (Windows 7), Adobe Flash wanted to download an update, and instead of waiting for the boot to finish as I normally would, I clicked on the Adobe Flash Download Update button. It then opened FF and hung, so I eventually used Task Manager to end FF. After the computer finished booting, when I opened FF it had no open tabs or groups, and the Restore Previous Session option (and each of the other options in that grouping in the History menu) was greyed. When investigating, I find I do have the sessionstore.js and sessionstore.bak files from yesterday (each about 4.5 mb), but I don't know how to access them because the Restore Previous Session option is greyed.