Try, as root,
ln -s /firefoxdir/firefox /usr/bin
You can then run firefox by simply typing firefox. This will also work for any other programs that you install.
If you want to add a new directory to your path, add the following lines to your ~/.bash_profile
When you log out and log back in again, the new directory will be part of your PATH, and you can run executables in that directory by just typing their name.
I hope this helps