You should download the tarball.
Then tar -xzvf <firefox file>.
Then, cd <firefox tarball output directory>
Then you should do a ./firefox-installer (or something close to that, do tab complete for the exact file name).
Then if it doesn't give you a link within your GUI, go to /usr/bin and there should be a firefox shell script, either execute sh <firefox shell file> at the command prompt, or find the file and double click on it.