No, there is no way to get it to version 5, but we'll look at patches if someone wants to jump start the effort. I don't have a firm grasp on which release it will fall under, otherwise.
What needs to be done to get the possibility of switching ECMAScript to 5 or 6 (Harmony)
As fare as I found out there is only one IDE out there, that supports Harmony, and this IDE is not free.
I'm really interested in using Eclipse for JavaScript development and for now ECMAScript 3 is fare away from being good enough to develop for Mozilla Addon SDK...