To install firefox 3.5 on ubuntu 9.08 if you can not handel it as usuall, you may need to edit /etc/apt/sources.list!
See
# type
#add following lines to the end of file:
deb http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu jaunty main
deb-src http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu jaunty main
# type
$ sudo apt-get update
# you will see "W: GPG error: http://ppa.launchpad.net jaunty
#Release: The following signatures couldn't be verified because t|#the
public key is not available: NO_PUBKEY #A6DCF7707EBC211F"
# type
$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 7EBC211F
#Now type
$ sudo apt-get install firefox-3.5
#;-)