Test Tabmix on clean profile, don't copy anything from your current profile
Ok, consider me confused now...
With a clean profile it works and yet it doesn't work.
I added a new, clean profile to FF. Then I disabled the .Net framework and the Nokia OVI add-ons (aren't those add-ons I can't uninstall a bug?), didn't touch the Java add-ons and installed TMP ...110402b. Then I started adding more and more tabs and saved a session from time to time.
During the whole process of adding more and more tabs I sometimes tried restoring the session (saved session, closed FF, waited until FF process vanished, started FF again and restored session after FF loaded). When I reached 140+ tabs I finally got a few not properly restored tabs when restoring the session. Upon a second try to verify no problem occured.
Then I started trying the session.rdf from my main profile which wouldn't restore the 230+ tabs session either. When switching back to the new session.rdf (140+ tabs) the restore went wrong again.
Upon further experimentation I tried clearing the disk cache of FF. After that restoring the session of the 140+ tabs went wrong. Each time I cleared the disk cache before restoring that session the restore goes wrong. When I don't clear the disk cache it seems to be a hit-and-miss issue somehow in my clean profile (with the 140+ tabs). The 230+ tabs session always has issues so far (I even upped the disk cache to 100MB).
What irks me the most is that in FF 3.6.15 everything works fine but in 3.6.16 this weird problem appears.
Thanks a lot for your time and patience (and, of course, such a great extension)!