You can use /opt or /usr/local. Obvously you will need to be root to install to these locations.
If you install firefox to /usr/local/firefox, you will need to create a symbolic link to the executable in /usr/bin
Code:
ln -s /usr/local/firefox/firefox /usr/bin
This is beacause the executable needs to be in your PATH so that you can type "firefox" in an xterm to start it.
I hope this helps, and welcom to LQ
--Ian