I've been playing around with firefox 3.5 recently (I got RC3 a few days ago).
The changes with the most impact for me are in the session store. I like that it lets you choose which windows to reopen after restoring from a crash, but there is still that "jeopardy moment" where you can press a button to destroy the old session, kind of like when pine asks you if you'd like to delete your old mail at the start of the month. The last version of firefox used a regular dialog box for this, so I'm sure I'll get used to it.
It also restores windows from other virtual desktops by minimizing them instead of dumping them all out to your current desktop. That's an improvement, but still not right!
The speed increase is quite noticeable to me (running Linux on an Intel Q6600) both in loading the initial session store, and watching text instantly reflow while resizing windows. The GUI is still not very responsive while it's loading ~70 tabs, but at least it doesn't take so long now.
I also crashed it once by clicking on a link from kartos
over AIM. I'm going to blame the flash plugin for that because I have no reason not to :P