Just manually upgraded to 3.5 and I'll give it a test spin, but be aware that 3.5 ISN'T compatible with many add-ons yet, including most of the ones I use!
I've never had to go back a version; I assume it's as simple as just reinstalling 3.0x right?
After using the new FF 3.5 for a bit I don't think I'll need to go back to the older version. FF 3.5 has a built-in zoom control that remembers the zoom level you set for the different sites you go to and zooms both text and graphics. Good Stuff! :T
Of the three video capture add-ons I have only Download Helper works, but it is the one I use the most anyway.
Sites like HTS load their advertisements MUCH more rapidly than before.
Have you tried to compare your download/upload speed with Firefox 3.5 vs Internet Explorer 8.0? Firefox is always has slower speed in my personal experience unless you do some changes in hidden menu or registry. I usually use Firefox but it's annoying that bunch of pluging required nowdays are not part of installation package.