You can run Firefox from its directory.
If you click on the script(looks like a file) "firefox" inside KDE it will start up. If you want to start it from the command line:
cd /path to firefox directory
the dot slash is used to run a bash script(program).
If you want to install firefox system wide go to the firefox help page on installing firefox. It will explain it a lot better than I can.
(I cannot post the below text as a link but it is the page you are looking for).