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AFAIK Pale Moon and Basilisk are the only XUL-based browsers left.

I don't think FF is using XUL anymore or at least not as much as they used to.

Even their docs for XUL shows it all as archived:

Firefox is actively moving away from XUL.

I thought they finished that recently when they announced the UI was no longer XUL based. Is there still more work to go?

I guess they are moving towards web standards? (like Vivaldi). I don't think they will ditch XUL in favor of native (C++) tech.