uncaught exception: java.net.BindException: Address already in use: Cannot bind

hi all,
i have using java to create a socket in java  and wrapt it to a firefox
extension .
when i open my firefox ,it's show the msg :
uncaught exception: java.net.BindException: Address already in use: Cannot
bind
but i find the port is using by firefox when i start it .
can i using java to create a socket on client side ?

any idea?