Updating to v35.0.1 lost all my open tabs

Question

I just updated to version 35.0.1, as suggested, and restarted. When FF reopened, all my open tabs -- 200 of them -- were gone. Sometimes problems like this resolve with a second restart, and I did that. No help. History -> Recently Closed Windows had only two minor entries. Keeping perspective, I won't call this a catastrophe; I reserve that for things like my home burning down. But it's a big loss. Some of what was open, I'll remember and find again. Most of it, I won't. I'll start looking through History, but some of these tabs have been open for a long time and I'll have to go back a long way, and look at many many tabs that I closed between then and now, to find the ones that were open before I did the update.
Can anyone suggest anything else I might try? I do a daily backup of my system drive. Are there FF files I could recover from backup that would bring back whatever tabs were open then.
I'd also like to know if anyone else had this problem.

Answer 1

I did some more research and recovered my tabs. But questions remain, and if this happens again I'd like to know as much as possible about it. I'd be grateful to anyone who can answer them.
This article led me toward the fix: https://support.mozilla.org/en-US/questions/1023996 . It gives these instructions:
o go to C:\Users\username\AppData\Roaming\Mozilla\Firefox\Profiles\somerandomnumber
o rightclick on sessionstore.js
o properties
o previous versions
o restore the version before you crashed
I found that directory but sessionstore.js wasn't there. Three other files whose names begin with sessionstore -- sessionstore-1.js, sessionstore.bak, and sessionstore.bak-20140923175406 -- were no help because they were, respectively, empty (0 bytes) and last updated in 2012, last updated on 2014-10-28, and last updated on 2014-09-24.
But also in the directory <somerandomnumber> is the subdirectory sessionstore-backups. It contains several files, the most recent one being upgrade.js-20150122214805. The time stamp in the file name is odd because it doesn't match the Properties information, which says it was created today, about the time I did the FF upgrade. But since the Properties info seemed right, I viewed the file with WordPad and it *was* the list of my open tabs just before I upgraded.
So I thought the fix was to close FF, copy upgrade.js-20150122214805, paste it into the <somerandomnumber> directory, rename it sessionstore.js, and restart FF. I closed FF, but as it closed, sessionstore.js appeared in the <somerandomnumber> directory. That was puzzling, but I viewed it, and it was the list of tabs I'd opened since upgrading and losing my old set of tabs. I couldn't restore a previous version because I don't have versioning enabled. So I saved that version of sessionstore.js under another name and created a new sessionstore.js as I described. I restarted FF, and glory be, my old tabs were back.
I watched the <somerandomnumber> directory again as FF restarted, and as it did, sessionstore.js disappeared again. To see if that behavior repeated, I exited FF, and as it closed, sessionstore.js reappeared. I opened FF again, and sessionstore.js disappeared again. Apparently FF moves that file somewhere else while it's running.
So my problem is fixed but several questions remain. I hope FF staff or someone with deep knowledge of FF can answer them.
1) Why did I lose my tabs during the upgrade to 35.0.1? Was it just me, just a fluke? Or has it also happened to others who upgraded to 35.0.1?
2) What created the three other sessionstore* files in the <somerandomnumber> directory? I'm pretty sure I didn't, and whatever did create them apparently doesn't do so consistently and in any case stopped doing so three months ago.
3) If those aren't the backups of sessionstore.js, where do those live? I've had plenty of FF crashes and System Restore has brought them back almost every time. Clearly they're stored somewhere.
4) What does FF do with sessionstore.js while it's running?
5) Why is the timestamp in 'upgrade.js-20150122214805' several days off the actual creation date and time correctly listed in Properties?