Lost Firefox "Restore Previous Session"

I had a brief power outage, less than a minute. When I restarted my computer (a 2012 Mac Pro with Sierra) and launched Firefox, my previous session didn’t come back. And under “History,” “Restore Previous Session” is grayed out. In previous power outages like this, I’ve always gotten an “Uh-oh” page from Firefox, or at least been able to restore the previous session via the pull-down menu. This time I’m apparently SOL. What happened?

In an orderly shutdown, Firefox is told to expect to be killed in the next second or two and therefore it ought to save whatever info it wants to. So it writes the session info onto disk for later use.

In a hard power failure, everything just dies. Firefox was just as surprised as the OS was and as you were. No status info was saved, so none was available later to read back.

Or perhaps there was a smidgen of warning and FF had started to save its data when suddenly its world went dark. And whatever got partially saved was garbled. In any case when the machine and OS & FF woke back up, FF wasn’t able to successfully restore state from the garbled data. It’s real unlikely the developers would bother trying very hard to cover for this massively rare situation.
Bottom line: relying on “save session” as a way to reopen all your daily favorites is not robust. Better to put a shortcut for each of your usual tabs into a folder in your favorites. That way you can choose “Open this folder in tabs” and they’ll all come back. And they’ll all survive random failures too. The downside is you need to keep those favorites up to date yourself.

I find it annoying when there’s a crash and Firefox is all bent over with apologies and wants feedback to it can avoid being such a disappointment in the future.

Just shaddup and let me get on with what I need to do.

Most likely the session data was being saved right at the time the power outage happened, and the data got corrupted. So then Firefox couldn’t read it.

You may want to look into using the Session Manager addon. It saves more than one backup session. Be sure to enable its crash recovery features in the options.

There’s also a Session Manager lite that comes with the Tab Mix Plus addon. It’s less powerful, but also has less settings.