Finally Firefox 13 officially available in Ubuntu 12.04.
It is available through standard repository ( update manager ) or you can download it from Mozilla website http://www.mozilla.org/en-US/firefox/new/
Firefox 13 also available for previous Ubuntu versions like 11.10, 11.04, 10.04 etc
Performance and Stability
Personally I was a Ubuntu user since 7.04 ( Buggy Windows Vista forced me to move on to Ubuntu. Thanks to Microsoft ! ). Since then Firefox is the official browser in Ubuntu. Looks much better in performance and stability. ( I faced a lot more issues in Firefox in previous versions )
The best thing I noticed is Web Developer tools. very nice tools for developers.
Tools –>Web Developer –> Web Console, Inspect, Error console, Style Editor etc
Try Firefox 13 today and share your experience.