The process I used to install Firefox 5 in Ubuntu 10.04:

1. Download the source from Firefox

2. Extract the package

cd ~/Downloads
tar xjf firefox-5.0.tar.bz2

Switches:
x : eXtract
j : deal with bzipped file
f : read from a file

3. Move to /opt

sudo mv firefox /opt/firefox

Note: I did not have a previous version installed. If you do you are going to want to move or rename the old version.

4. Setup Symbolic Link
>Rename the old launcher so you can still use if needed<

sudo mv /usr/bin/firefox /usr/bin/firefox-old

>Create new symbolic link<

sudo ln -s /opt/firefox/firefox /usr/bin/firefox