I've recently started having a problem with Firefox 3.5RC2 (on Mac OS 10.5.7): it hangs and stops loading pages after awhile. Sometimes, it's after an hour, and sometimes, it's after 2 minutes. What's strange is that I can then launch Safari and access *any* page, and then Firefox will resume loading pages. I've disabled all my add-ons, and it hasn't resolved the problem. The next step for debugging will be to trash *~/Library/Application Support/Firefox* and start a completely new profile.
Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1) Gecko/20090616 Firefox/3.5