I don't know why you don't see this in 3.3.x, but I observed this from the very beginning. The Firefox bug you reported was already in 3.2.x when CKEditor was introduced. I recently upgraded from 3.2.3 to 3.4.3 and I don't see any difference in Firefox behaviour.
Wierd - I went from 3.14 to 3.3.1 and have never had this problem pasting in FF in the IPB editor nor have any of my users.
Anyway, its such a basic requirement for an editor and FF has such a significant user base, that I'm simply amazed an editor is being used with such a significant usability glitch.
Just looked at my post and it says 3.49 undefined in the quote - where on earth did that come from????