We should not open the context menu if right click or right mousedown called e.preventDefault or returned false. Obviously in that case we should not fire oncontextmenu.
*** Bug 131960 has been marked as a duplicate of this bug. ***
This bug prevents users from using their favorite mouse tool like MouseImp Pro. The users expect scrolling window on right mousedown, but have the context menu instead. The same relates to Thunderbird windows.
There was no such issue in Firefox3