It sounds you're using Ubuntu 10.04 Lucid since the latest version (11.04 Natty) is supplied with Firefox 5. Otherwise, remove your older Firefox packages and install Firefox 5 from the official repository.
Instructions for Ubuntu 10.04 Lucid and Ubuntu 10.10 Maverick (the proper way)
Instead of installing the tarball, you're recommended to install the
mozillateam/firefox-stable PPA and use
firefox from that package (see also What are PPAs and how to use them?, Lucid-specific information is available here).
This PPA is maintained by the same guy responsible for the
firefox package in the
main repository. PPA description:
Firefox Stable releases.
Aimed at Lucid and Maverick users who want to use a modern browser which starts faster than you can brew your morning coffee
Shell commands for adding the PPA, refreshing the repository information and installing Firefox 5:
sudo add-apt-repository ppa:mozillateam/firefox-stable
sudo apt-get update
sudo apt-get install firefox
Installing from tarball
If you still want to install the 32-bit tarball (there is not 64-bit version available from Mozilla), install it into your home directory. Instructions can be found on a Askubuntu question on installing multiple Firefox versions.
If you've a multi-user system for which you do not want to install Firefox each time, put the folders in
/opt since it's a stand-alone package which do not follow the directory layout that is suitable for
/usr/local. Plugins should be put in or
~/.mozilla/plugins (for per-user installations) or