I doubt if Sync will help in that situation, Sync wouldn't have been able to "sync" that file to the Sync server. Plus, Sync uses the same file for storing sessions that Firefox uses; if that file is corrupt due to Firefox getting closed improperly (computer getting shutoff due to power failure), that file would be corrupt for "sync", too.

http://en.wikipedia.org/wiki/Uninterruptible_power_supply - less than $50 when you but one on sale

So i need to buy a new UPS to restore my previous session? Gee, thanks.

You're being silly now.

If Firefox crashes, your session might not be saved or the file session data is saved in might be incomplete or could be corrupted. The point is prevent Firefox from not closing properly.

You're being useless now. I did not ask for an advice on how to prevent Firefox from not closing properly.

averagefailguy,

Hey, no need to be rude.

Your question was:

I had Sync on at all times, would this help? This pc is my only synced device, though.

As the-edmeister pointed out, no, you can't. If Firefox closed improperly or was forced to shut down due to a power outage, Sync probably wouldn't have synced it over and if it did, it was likely corrupt.

You said Firefox crashed? There may be some crash reports left over to see what caused the crash
EDIT: Forgot Firefox didn't crash but force closed.