First, thanks everyone so much for your time and replies.
If I enter about:sessionrestore, I'm getting the message:
"Well, this is embarrassing.
"Aurora is having trouble recovering your windows and tabs. This is
usually caused by a recently opened web page,"
and there are no windows or tabs listed to restore, even though I still
have many of each open, and sessionstore.js is still 18 MB. Goodwin,
what you say seems to makes sense, that it's empty because everything is
normal, or rather that there wasn't a crash to restore from.
For what it's worth, I don't necessarily want so many windows and tabs
open, but I always end up working on many things and getting interrupted
to work on more things and having to switch back and forth. I can't be
the only one who has to work on so many things at once like that.
I, too, wonder if the cause could be a corrupt file, but I'd like to be
able to not have to lose all of that session data - to retrace so many
steps - so I wondered if there were a way to figure it out the cause
and/or "clean" the file or something. Oh, and regarding the cause being
a lot of tabs open, I do wonder about that, of course, but I do
regularly close windows and tabs, not just open them. The number of them
does go down, but sessionstore.js doesn't get smaller, it just keeps
getting larger anyway.