return false

[Surrendermonkey]

I have oncontextmenu="return false;" and oncontextmenu="javascript:return false;" as attributes (it's a <TH> tag, incidentally, but I don't suppose it matters), and the resultant behaviour is satisfactory. However, when I have oncontextmenu="javascriptoThing();" along with
function doThing(){
return false;
}
I still get the context menu popping up.
Why is this, and how can I prevent it?

thanks,

S. Monkey (Mr)