Not long ago, Mozilla released Firefox 3.6 (Codename: Namoroka). But while Firefox 3.5's successor is being tested by developers, another future version, Firefox 3.7 was made available at download sites.
If you want to try Firefox 3.7 for testing purposes, although the nightly build means it's not a full-fledged release, click this link.
Here's another link where you can download Firefox 3.7 nightly builds directly from Mozilla servers. Click here.