I have this problem too and don't say my system is slow.. haha.. Specs below..
There seems to some kind of problem with the session restore once you load the browser. Having many open tabs previously seems to directly related to the problem. On my home PC it just takes a while to load on my office notebook where i always open lots of tabs... after a long wait I get an error stating the previous session window cannot be restored.
I have no such problem with the previous release. FF 3.5 is certainly buggy, hope the have a patch soon for these performance problems.
