A recent message on the Ubuntu development announce mailing list points out that Firefox 3.6.4 is about to be pushed in Ubuntu Hardy, Jaunty and Karmic (and Lucid - obviously):
Next week, Mozilla will release Firefox 3.6.4 as a minor update to the 3.6 series. This will be rolled out to Lucid, Hardy, Jaunty and Karmic (along with xulrunner 184.108.40.206).
Until now, users which didn't use the latest Ubuntu version found it to be a real pain to upgrade to the latest Firefox version.
The major reason for this is that Firefox 3.0 is not supported by Mozilla anymore so the Ubuntu developers would have to backport security fixes to Firefox 3.0 in Ubuntu Hardy, Intrepid, Jaunty and Karmic. Also, Mozilla seems to be releasing new versions faster than they used to, so Ubuntu needs a new way to upgrade Firefox in all of its versions.
However, this change has some implications as the latest Firefox depends on a newer xulrunner, meaning applications which use it must either switch to webkit or be updated to the latest xulrunner version. Hopefully everything will go well.
Until the Firefox 3.6.4 will be officially released in all supported Ubuntu versions, there is a PPA you can use to test it which you can find HERE.