According to the latest news from the Mozilla folks, the nightly builds for Firefox now include support for VoiceOver in Mac OS X. I have downloaded it, and though it is clearly in its early stages, it’s really exciting and pleasing to see the effort these folks are putting into accessibility. I will post a link to the blog post here and keep in mind the performance issues Marko points out. I would strongly recommend downloading it and really giving it a good run, reporting as many bugs as you can find so they can be dealt with.
Go here for the blog post and all of the information, including a download link.