Get it
here, or do a google search for 'mozilla mirror' if you find the main site busy.
It might be worth checking out your extensions first, and seeing which ones won't run with this release.
FlashGot updated itself to 0.5.0, Close Tab on Double Click was replaced with Tab Clicking Options (from the same people/homepage, but it needs to be done manually) and Single Window doesn't have a 1.0 version out yet. 2/3 aint bad I guess, and to be quite honest, Single Window didn't work all of the time anyway - it had some rather weird inconsistencies. Note that there may be an updated Single Window on update.mozilla.org, but the site simply won't load for me at the moment, so there's no way to find out..