Seems to me like their ancient fork of firefox doesn't work correctly on github, so they are moving.

> doesn't work correctly on GitHub

There is already extension[0] for Pale Moon which solves issue with GitHub.[1]

[0] https://github.com/JustOff/github-wc-polyfill/releases

[1] https://forum.palemoon.org/viewtopic.php?f=70&t=25435

> so they are moving.

Mirror would be available on GitHub anyway:

> We (or rather Tobin, specifically) will maintain a mirrored repository for the platform on GitHub.

Well, they shouldn't have moved before that was ready. As of now there's no Palemoon mirror at https://github.com/MoonchildProductions

I was going to see if perhaps the abuse of Issues was what annoyed the maintainer more than the primary reason).

That's one of big downsides of a world where 99.99% of people create nothing (in terms of code or docs): when Github wasn't as popular many "consumers" didn't have accounts.

Now they do, and often tend to turn Issues into a discussion forum (which, contrary to Tobin's assessment, is a testament to the success of Github's UX, but not of the kind devs and power users want). The easiest way to deal with that is what Tobin did. Mirror the source to Github and lock (or ignore) GH issues.