Getting xchat to open Firefox instead of whatever

It’s covered in the XChat FAQ: What’s the deal with opening URLs in XChat on Unix/Linux?.

I manually added the “Open link in Firefox” option, and haven’t bothered figuring out how it works on Edgy’s Xchat (2.6.6), and decided to wait and see how things work in Feisty (Xchat 2.8.0), as the FAQ says everything’s changed in 2.8.0.

Update: I’m using Ubuntu Gutsy, which has XChat 2.8.4, and everything works fine by default, without having to tweak anything.