We've decided to update XKit. It's harder than it looks. We're not in contact with the original XKit Guy, nor do we have access to xkit.info. We currently operate at this blog and on GitHub.
just develop for Android. it doesn't cost money. you could even just make a version of xkit that works with tumblr's mobile site and people could use that with Firefox.
People already can install the extension if they use Firefox on an Android device.