I don't use Mozilla, I use the Firebird version (browser only). I have found a link with things that explain the stuff behind web-editing with Mozilla.
One question though: Why doesn't it work with vB? All a WYSIWYG editor does is show the page with the rendering engine (for example, Frontpage uses the IE engine and Mozilla Composer uses the gecko (=Mozilla and Netscape) engine), with a few extensions.
Not sure why it doesnt work. I have Mozilla 1.4.1 and I'm running vB3 beta 5 privatly right now and it doesnt show. The devs said they are working on getting it to work. Which is good, because it looks awesome in IE.
The WYSIWYG editor does work in Mozilla (I just found that out). vBulletin 3 was developed from scratch, they didnt just enhance the code from vBulletin 2.x.x. All of their HTML is XHTML compliant which is great news. vB is still in beta stages, and they just said the WYSIWYG bar is not currently working in Mozilla, but the next beta should be.