This is what you are supposed to have ...
dpkg --get-selections | grep firefox
: firefox-3.1 : dummy upgrade package 3.1 -> 3.5.2
: firefox-3.5 : package manager says 3.5.2 ... to-day ..
dpkg -l firefox-3.5 :
firefox-3.5 .. 3.5.2+nobinonly safe and easy web browser
( I had to change mirror to http://ftp-stud.fht-esslingen.de
to get it.)
.. So you can just update to that with
sudo apt-get install firefox-3.5
... And .. to-morrow ... you may get version 3.5.3 ,
already available in e.g. the Fedora 11 repo.
I don't know how slow an delayed Ubuntu is ! !
3.5.3 is better : checks and warns about unsafe flashplayer.
EDIT EDIT : The dummy upgrade package 'firefox-3.1' can with
no problems be uninstalled > > Then may be no more upgrade ?