For me personally, the only reason I downgraded back to 3.0 from 3.5 is because Firebug 1.4 is bloody awful at the moment (in beta). Other than that, Firefox 3.5 is great.
Other than that I suppose faster javascript and optimisations through use of sqlite for history/bookmarks is good.
Probably a few other things if I thought about it.
Main features are in add-ons for me, firebug, yslow, noscript, adblock, seoquake,download toolbar, greasemonkey.
I keep a separate clean profile, without all the footer-bar ("clutter bar") icons for the wife to use whilst surfing
* Bookmarks. I can easily bookmark any page by clicking on the star next to the address bar. Clicking it again allows me to edit it or remove it. I use this all the time.
* History with search.
* Spell check. I used to think it's useless but now I cannot live without it.