Just a heads up: Since Firefox download sites are clogged today, you may be tempted to use an alternate URL you see posted somewhere on the web in order to get the latest version. Be careful, though, because I've seen at least one of these links lead to a purported spyware program instead of a browser. Windows users should be especially vigilant.
According to Mozilla, "Firefox 3 is currently only being offered as a full download from http://getfirefox.com/". It might take you longer to get it from there, but if you want to be safe I recommend you stick with the official site, or at least some site that you know and trust.