Hi Jamespole and welcome to the Arch Wiki!
Thank you for your edit to Firefox/Tweaks. We appreciate everyone trying to improve the wiki!
For the moment however, I have undone your edit, as I'm not sure how the previous instructions are "out of date": I just tried them in Firefox 57 and they still work fine. Besides that, the old instructions seem superior to me, as
network.protocol-handler.expose.magnet only influences the handling of magnet links in Firefox (which is what the section is about), whereas
network.protocol-handler.expose-all influences all protocols.