Despite the fact that Firefox 13 isn't scheduled for full release until tomorrow, the final version appears to have leaked out for Windows, Mac, and Linux users. As we saw when the beta for version 13 was released back in April, there aren't a ton of user-facing changes here. The biggest change is probably that Firefox no longer automatically restores all tabs when starting up — a feature that some might find useful, but some could also find quite frustrating. Fortunately, it's still easy enough to pull up all tabs from your previous session in the history menu.
There's also a new "home" screen that greets you when opening the browser that provides easy access to your downloads, bookmarks, previous session, history, sync settings, and general app settings. Lastly, opening a new tab brings up a grid view of commonly-visited sites, just like Safari and Chrome have done for years now. As for under-the-hood changes, the biggest is probably the SPDY protocol being turned on by default, which should help overall performance. If you want to give the latest Firefox a shot, check out the release notes here (for the beta) or just download the new version at the links below.