firefox and pptp vpn

i tested many ways to know this things !
totally when i use vpn connection (pptp) youtube movies when they loaded 50% i close firefox window but my firewall still shows me a program, connection or .... that using all of my internet bandwidth
i tested my vpn connection with IE and eveyrthing was ok
then i installed traffic comp program
i just activated vpn and then vpn connection and everything was ok even with firefox!
it means when i went to youtube and select a trailer when it loaded 20% i closed the firefox and all of my internet bandwidth became free
then i tested it again without traffic comp and again that problem
after i close my firefox window (youtube site) still that movie loading and all of my internet bandwidth using by a firefox
i don't know but i think this is something between firefox and vpn connection
i think maybe firefox don't get stop command through vpn

any idea?

it is Traffic Compressor
known for poxy features and also interet traffic moderator