I take it you downloaded a zip file called firefox-riscpc_18.104.22.168-2.zip, opened it and found three directories inside?
Within the zip file, the file Apps/Network/!Firefox/!Help contains some instructions. I think you should be able to just drag and drop Apps/Network/!Firefox to a directory somewhere, then access the help using the filer menu.
Go to the section "I just want to run it", where it tells you about a couple of things to merge into !System before you can run the browser.
That's the best I can do without the kit to try it out myself. Please, tell us how you got on!