Rated 5 out of 5 stars

I use it as extra backuptool just in case FF or my computer crashes and restoring the old session goes wrong. You want to reopen your current session weeks, months or even years later, for what reason ever? Here you go. And it's a wonderful help, when you're abusing open tabs as temporal bookmarks/"wannaread"-reminders and you have to save them somehow.
As far as I know the gest session manager around for FF. (I can't tell if there is a better one. Never felt like looking for an alternative.)

***Now here's a little request aimed at the developer: It says, that it works with FF3.7a2pre. Now that FF3.7 or what*'s left off it has become FF4.0 and FF4.0b1 is out, how about checking, if it's compatible?

This review is for a previous version of the add-on (0.6.8). 

0.6.7.4 is not FF4.0 compatibility. The just submitted 0.6.8 is compatible with FF4.0. I expect it will be approved within a week or two, though it might take longer. If you're impatient you can install it now in the "All Versions" page.