Rather than dealing with back-porting security changes to previous branches of Firefox, new versions of Firefox will simply be pulled, which will provide the security fixes along with all of the new features too. This change has been in the works for a number of months and with the Ubuntu 10.04 "Lucid" version of Firefox it's much easier to handle the transition to future versions of Firefox. This Firefox upgrade model is also similar to how Google's Chrome / Chromium will end up working too. All of the details about this new Firefox support model within Ubuntu can be found on the Ubuntu Wiki.
There is also a new message on the Ubuntu development list in regards to these latest efforts. Next week upon the release of Firefox 3.6.4, this new version will be pushed back into the Lucid, Karmic, Jaunty, Hardy releases of Ubuntu to replace their existing versions, which in the case of the older releases are Firefox 3.0. Right now the development community is looking for people on these older releases (or running them within a VM) to test out their packages of these new Firefox binaries.