either put it in /usr/bin/firefox or /usr/local/bin/firefox.
Remember to create a symlink in /usr/bin or /usr/local/bin to firefox's binary. For eg.
firefox -> /usr/bin/firefox-1.5/firefox
isnt /usr/local/bin and /usr/bin deprecated for additional software ? the standard these days seem to be /opt more commonly used.
-- Dinesh A. Joshi