I just updated my main machine to Firefox 3.5. No major problems, but a few speed bumps.
Google Gears told me it didn’t work with Firefox 3.5 and there wasn’t an update available. I uninstalled the current add-on and installed the latest version from gears.google.com and all was well.
I had the same problem with the Evernote clipper add-on. Firefox 3.5 said it was incompatible and no update was available. An uninstall and install of the latest version resolved the problem.
Firefox 3.5 is supposed to be faster than version 3. I’ve been running it on Windows where Firefox has always been faster than on my Mac. I can’t say it’s noticeably faster on Windows. But some of that may be because I use noscript to keep Java from running except when I want it. The improvements are probably more noticeable on sites with Java and other scripting.