Firefox stays active in memory after closing program

After closing Firefox and restarting the progam after a while, a popup message appears saying Firefox is allready active (message in dutch) and Firefox does not (re)start. It is not an active window however, since I closed it down... When I press alt-ctr-del, Firefox can be found as "active"in the processes-tab. To re-start Firefox I now manually must click on "end process" and now Firefox starts without problem. This happens on each occasion after closing the program. Anyone know how to solve it? Help appreciated!