Mozilla has released Firefox 13 on June 5th, 2012. Several UI changes in this update and more information can be found here and in the Release Notes. This release does not support Windows 2000 or Windows XP SP1 (or older) due to the move to MSVC 2010 compiler for the builds. Users can update via Help > About Firefox or can manually download and install the new version of Firefox via getfirefox.com site. Firefox 14 will be the next release in six weeks (July 17th).