Just a specification: once uncompressed and unpacked the tar.gz file, there is nothing more to be instaledl! Firefox is already there and needs only to be launched. This has a main consequence: Firefox is exactly where you unpack the tar file. Thus, if you unpack it in your home directory, the firefox path will be: $HOME/firefox/firefox. If you are root and you want to put it at disposal of each user, unpack it otherwhere: e.g. /usr/local/firefox
Cheers
Giorgio