Topic.
You can download the Firefox source code, make the necessary changes to allow the javascript and stuff thats needed to support it. Compile your source and release the new package to the open source community. :)
Seriously though, why are you trying to do this on a product that's not even beta? It already works on FF3.
The easiest thing to do would probably be just to change your useragent. I don't think firefox 3.1 will have many major rendering changes so it should work. I believe 4.0 is having more js improvements though
Unfortunately, I am not satisfied with these answers. I remember in the past that one could edit the useragents SMF recognized. Can I still do this? If so, how?
bump
Does the WYSIWYG editor not working for FF 3.1/4.0.
The source code I quickly glanced at doesn't look like it restricts any version of Firefox for the WYSIWYG editor.