I can think of several reasons, namely that most security updates in Firefox are for Windows-specific issues, and that with Firefox's new rapid release cycle, there is less time to certify a release. Nor have there been any major feature additions to Firefox that would warrant rushing it in.
If you are really anxious to use the latest Firefox, you can always add this PPA:
http://launchpad.net/~ubuntu-mozilla-security/+archive/ppa
EDIT: Java isn't even included in Firefox, so it's impossible for them to have improved "the Java rendering engine." Perhaps you meant JavaScript? All releases of Firefox claim to be "30% faster"; the real world differences are usually minuscule.
http://www.reuters.com/article/2009/07/01/urnidgns852573c400693880002575e5008290bb-idUS55438673720090701
http://www.webmonkey.com/2010/01/firefox_has_been_hitting_the_gym_-_version_3dot6_is_faster__more_capable/
http://www.infopackets.com/news/internet/2011/20110323_firefox_4_released_6x_faster_lots_of_changes.htm
http://www.thriftmac.com/firefox-5-faster-than-ever
http://ulyssesonline.com/2011/08/15/firefox-6-is-20-faster/
http://venturebeat.com/2011/09/27/firefox-memory/
http://www.extremetech.com/internet/89570-firefox-8-is-20-faster-than-firefox-5-matches-chrome-14
http://betanews.com/2011/11/10/whats-coming-in-firefox-9-10-or-11-little-to-get-excited-about/
The page I linked to earlier (Yahoo cropped the link for some reason so I've modified it above) show you how to add the PPA. It takes exactly three lines of copypasta. You can just copy the following commands below.
1. Open a terminal.
2. Enter
sudo add-apt repository ppa:ubuntu-mozilla-security/ppa
3. Enter
sudo apt-get update
4. Enter
sudo apt-get dist-upgrade
That's it. No Mensa membership required.