>slightly fewer vulnerabilities and stability issues than 3.5.<
What stability issues? Win7 has crashed more times than FF3.5 for me. Mind you, I tend to only have 7 or so required plugins and never more than 10 tabs open at the same time, of which I close those I'm finished with. I also close and restart FF as and when I need it, and whilst I think the memory footprint is excessive, it doesn't seem much worse than the other browsers out there (I've tried Opera, Chrome, IE 6, 7, 8 - but always return to Firefox, for purely personal preferences, I wouldn't say any one browser is better than another, and IE8 is leaps and bounds better than its predecessors).
As for vulnerabilities, isn't that part and parcel of being windows based? Java and Flash basically open the back door to your system, use NoScript (only available for FF) or a decent firewall, I've heard no reports of Ubuntu FF being easily hacked, but I'm not sure I didn't misunderstood your original point (double negatives FTW).
My one complaint about FF is the UI is too small (touch screen lappy here y'all) by default, but even that can be changed by the user by simply editing text files.
In conclusion, I like, very much, Firefox 3.5, looking forward to .6 and V4 looks to be heading in a good direction, UI wise.