The irony is what use to be its greatest flaw, memory usage/leaks, is now it's greatest strength. Around Chrome 15+ memory usage started ballooning and was too much for my 2GB netbook. A laggy UI is much better than an unresponsive system from thrashing RAM.
I've run into some strange behavior like this before too. When you reopen Firefox and its lost your session, try opening another window or two and see if 'Restore Previous Session' is finally available.
If that didn't work, there are also a few other tools that can help solve restoring your sessionstore.js:
Firefox Sessionstore.js fixer
http://thepanz.netsons.org/post/firefox-sessionstore-js-fixe...
What if Firefox loses your tabs ?
[0] https://people.mozilla.com/~shorlander/ux-presentation/ux-pr...
This is all on a Windows 7 machine though, rather than OSX.