Two options come to mind right off the bat. First, firefox isn't in your $PATH as regular user, second you don't have execute permissions on firefox.
Open up a terminal then:
# whereis firefox
This will tell you where the firefox binary is (in my case it's /usr/bin/firefox). Now ls -l on that path and check the permissions line to make sure that all users have execute permission. In my case:
# ls -l /usr/bin/firefox
-rwxr-xr-x. 1 root root 4715 2009-12-16 11:25 /usr/bin/firefox
You need to make sure that the last three are set to r-x. If not, run:
# chmod 755 /usr/bin/firefox
Second theory is that it's not in your current path. Run:
and check to make sure that the location (/usr/bin) is listed in that output. If not you need to add it: (as user)
$ export PATH
Then you need to add those lines to the appropriate set up file (like ~/.profile) depending on your shell.