We still support targeting Windows XP as a platform, but it’s always been a bit… odd. We don’t make the same guarantees about it as other platforms.
Firefox 52 ESR was the last version to support XP and Vista. Current Firefox ESR is 60.
If we wanted to be aggressive, we could drop support for Vista as well, it has smaller market share (from what I’ve read) than XP, but in my understanding, it’s good enough to be a full platform, and so there’s less reason to drop it.