A complete absence of the editor suggests a TinyMCE incompatibility. I'll take a look to check that it's not something we're doing in XenForo, but we're not likely to spend a lot of time on it while Firefox 9 is still in beta.
I'd be more concerned if this were a stable version of the browser, but it's called Beta for a reason, and there have been a lot of JS changes from what I've read. I don't think there's anything we can do about it, but I'm not going to chase it around with a beta version of the browser.
This does seem like a browser issue -- it looks like FF9 introduced a new console object (or new code for it), and it kills JS internally if given a bad variable (forced to display as an object). If you have Firebug enabled, you won't see this issue.