open task manager, click on the processes tab, look for firefox in the list, click once to highlight it, then click end task....
The task manager can be opened a few ways, on of them is to right click on an empty part of the task bar at the bottom of the screen and select "task Manager"
You are not supposed to notice this kind of problem with Firefox. Posting anyting about it online is the ultimate disrespect of "the best browser". Shame on you.