What older versions did you use?
Here are some pros: better security (much better that IE8); better support for newer versions of html, CSS, etc; better javascript engine; here are some addons: download status bar, downloadthemall!, built in IRC client (don't remember the name now), FireFTP, AdBlock Plus, Noscript, VideoDownload helper, etc (there's something for anyone!) + themes.
Here are some cons: it can use a bit more memory that IE (especially if you have many addons), if you have many addons loading times increase (i only keep 3 or 4 at max just to be sure) - no other cons i can think of right now. FF 3.6 solves most of the loading times problems but i'm waiting for the final release not the beta