Rated 5 out of 5 stars
As you have already said, due to breaking changes the theme might be as well written from scratch, but do we really need that? Releasing the source to the public (not in the form of xpi-obfuscated zip archive, but a github/bitbucket repo) could save a lot of hassle both for you and your users. Many of those would love to contribute and they probably already can by sending you another email with a zip of the corrected source code, but there's a cleaner and more fun way :)
The source code within the xpi is not obfuscated (if you have an obfuscated one I'd guess someone put adware or other malicious stuff in it and tries to hide the fact, but I guess/hope you just didn't open it up?). I don't see how investing time to bring it to some of these platforms helps the theme, though; as I could also use that time to work on the update. A public git repo means that I'd feel the need to think way more about repository structure, build system, etc. I don't think publishing a repository without build instructions and proper documentation is a good idea.
Oh, and now reading your review's title: The source code is under MPL 2.0 (http://www.mozilla.org/MPL/2.0/), so you can fork it and/or send me patches without any legal hassle.
Edit2: for extended discussion, please use eMail; this rating-reply interface is not suited for discussions ;)