Correct, which completely disproves the idea that FF adoption is in any meaningful way driven by (the lack of) XUL support.

If that was true, we'd expect to see forks like Pale Moon be _way_ more popular.

I disagree. The XUL forks serve their purpose well, but their lack of popularity is more due to being built by a very small team of volunteers, who've taken on the thankless and gargantuan task of maintaining the aging codebases of both Firefox and XUL addons, backporting security fixes from upstream, all the while fighting against limitations imposed by Mozilla as mainstream Firefox moved on.

There's no reasonable chance that such a project could ever build a user base to register as even a blip on the market share graphs.

Some people avoid it precisely because of this doomed factor. Personally, I can't rely on nor trust that a project of that scale is able to navigate the security minefield that modern browsers are, and fix all the routinely discovered issues, let alone those that remain hidden and unreported. So choosing to use it is a conscious acknowledgement that you're sacrificing security for those other features, and very few people are willing to make that sacrifice.

The premise in the comment that started this chain is that Firefox's declining market share is caused by a lack of XUL support.

The only way this can be true, but for Pale Moon and similar to have almost no userbase, is if the majority of people are leaving FF for lack of XUL support and going to Chrome/Safari, which makes little sense to me.

People don't know about Palemoon or Seamonkey or Konqueror or other web browsers. They knew about Firefox because it evolved from Netscape -> Mozilla.

Again, this is completely irrelevant to the point that I'm making. It would be possible for people to find out about them if they actually cared about XUL support so much they quit using FF for it.

Well, i use PaleMoon. Firefox is only for sites which don't work with Palemoon.