You need to add the firefox directory to your system path. Under Control Panel / System, go to the Advanced tab and click "Environmental Variables." If you want to do this for all users, look under System variables; if you want to do it just for yourself, look for User variables. Find the variable called "PATH" and edit it. Add "C:\Program Files\Mozilla Firefox" (or wherever Firefox is installed) to the string (semicolon delimited) then click OK.