To add onto FreewheelinFrank's reply,
As Firefox and Chrome are open-sourced, it is only natural that more bugs are to be found; but because it is open-sourced, they will be fixed by the community. Internet Explorer, on the other hand, isn't open sourced. So naturally, less bugs will be announced to the public domain. The bad guys would want to keep the vulnerability from going public for as long as possible, no?
~!Donovan