How to change bookmarks in Firefox shortcuts with right mouse button?
If you select 'Show All Bookmarks', the Bookmark Library will open.  You can then right-click there and change.

You are quite right in that you cannot right click a menu item in OS X and have it perform a secondary function (not in any of the major browsers anyway).

So the quick way to achieve what you want is - Shift-Command B to bring up the Bookmark Library.