for more info on the menu mouse down thing, consult this bug: http://bugzilla.mozilla.org/show_bug.cgi?id=49844
Some memorable quotes:
"I like it firing on mouse down. that's the way god intended it to be."
"This context menu on mouseup thing is a big pain in the butt for the many people, and big turnoff from mozilla."
"there are features of using mouseup, there are reasons we don't use mousedown and these were discussed in other bugs. please take your complaints to a newsgroup."
"we are using the win32 WM_CONTEXTMENU event. Period. Done. This conversation is over."
Eventually, we'll be able to hijack the mouse click events and display the menu at the correct time. until then, we just have to live with this.
I actually have a version of kmeleon with the menu appearing on mouse down, but it requires a hacked version of Gecko...