Firefox demon & strange processor load

Lately I have very strange thing with my firefox browser. After that I close firefox I still see it running in process list and consuming a lot of system resources. Currently I`m running window vista so probably that the main reason of this strange behavior. On windows xp I never had such a experience.

Op.System: Vista Business
Browser:Firfox 3.01
Plugins: Firebug, Firecookie,Liverank