Good night from France,
i'm using the Firefox 1.5 i've just installed manually in my /usr/local/ directory hierarchy because i have not found it yet as package for the latest stable ubuntu.
First, you need to download the firefox archive file on Get Firefox
After downloading the needed file, it was very simple because it's just a matter of executing only 3 commands :
tar -xzvf firefox-1.5.tar.gz
sudo mv firefox /usr/local/
sudo ln -s ../firefox/firefox /usr/local/bin/
That's all folks !
Because /usr/local/bin is before /usr/bin in the default user PATH shell variable you don't have to modify your default firefox icon on the gnome launch toolbar. The default firefox launching shortcut points to the "firefox" command so it will the "/usr/local/bin/firefox" before the packaged "/usr/bin/firefox" command.
Have an happy new year and happy using of you newly locally installed Firefoxof the next generation than 1.0