Why does QuickTime lag in Firefox if I don't put my mouse over it?

Why does QuickTime lag in Firefox if I don't put my mouse over it?

This has happened for me as long as I can remember. Since the first version of Firefox, on multiple computers and under different versions of Windows. QuickTime plays fine in IE and Chrome (even with Firefox in the background), but in Firefox if my mouse is not over the QuickTime window then it will start to studder, then lag and eventually just stop.