Firefox keeps its application data in ~/.mozilla/firefox, and a given profile may only be used by one running instance of Firefox at a time. Because CS Linux machines source home directories from the same server, this means one user cannot run Firefox on more than one computer. If you encounter an error message claiming Firefox is already running when launching it, this means your account has Firefox running on another machine. In order to use Firefox, you must find the other machine(s) and quit Firefox.
In order to avoid this occurrence, please log out of lab machines when finished using them.