You should put firefox & any other programs that you install in the same directory (For consistency). Most people use /usr/local, or /opt.
You will need to create symbolic links to the executables in /usr/bin, so that all users can run them. For example "ln -s /opt/firefox/firefox /usr/bin".
If you want to install programs for just a user, it must be installed in that users home directory, I use /home/ian/prog for this.
I hope this helps