Rated 3 out of 5 stars
This add-on does exactly what I want, except that it interferes with Firefox's session restore.
If I set “When Firefox starts: Show my windows and tabs from last time”, then when I close and reopen Firefox I just have a lot of blank tabs. This only happens when this extension is active.
Very useful add-on — would be 5 stars if not for that bug.
Edit: This bug is now fixed in 0.8.3.
Edit 2: I'm seeing this bug again with 0.8.3. (Max tabs is set to 100; could it be that this bug is only triggered once the total number of tabs in the session exceeds this number?)
Should be fixed in 0.8.3.