I just updated the version of Firefox to Firefox 3.5 in one of my virtual machines.
It sucks.
The problem is this: Every time I load a page in a background tab, anytime Firefox needs to process Javascript in that page or whatever, Firefox freezes in the foreground tab; anything I type in (such as replying to an email) isn't typed in and I can't scroll the page or do anything else while Firefox processes the background tab.
Firefox 3.0 doesn't have this problem, so I'm going to use Firefox 3.0 until Firefox stops making security updates for Firefox 3.0; Firefox 3.0.11 can still be downloaded here ("en-US" is US English and "es-AR" is Spanish) and will be updated with security updates for about six more months. After that, well, hopefully the Mozilla foundation will solve their Firefox 3.5 issues or I will switch to another browser.
Firefox's big advantage is that it has a "always use the Verdana font" option, which no other modern browser has; I was able to hack this in to Opera in my Linux virtual machine by having the only available True Type font be Verdana.