Selenium RC "java.lang.RuntimeException: Firefox couldn't be found in the path!" problem

Hi friends,
I am new to selenium RC using php, I am working in Fedora core 12. When try to run the Test I got

11:49:22.380 WARN - GET /selenium-server/driver/?cmd=getNewBrowserSession&1=%2Afirefox&2=http%3A%2F%2Fwww.google.com%2F HTTP/1.0
java.lang.RuntimeException: Firefox couldn't be found in the path!
Please add the directory containing 'firefox-bin' to your PATH environment
variable, or explicitly specify a path to Firefox like this:
*firefox /blah/blah/firefox-bin

on that time I give the path of firefox as "usr/bin/firefox" then got an error like it's not an executable file. I heard solution
need to downgrade Firefox from version 3 to 2. But same thing work in Fedora Core 13 system like a Charm.
pls give a solution ASAP.

thankfully
Anes P.A