Magnet links on Nightlies

Open about:config page;
type in network.protocol-handler.expose.magnet as boolean value set to 'false',
you're good to go. It'll gives you protocol association windows the next time you open magnet links.

IMHO, It's definitely the easiest way to associate new protocol in later versions of Firefox (which come with confusing rapid release scheme in the package :D ).

Thanks to ArchWiki.
 ____
Anyway, I've never really liked the idea about Firefox's rapid release development: just knowing your build number is bigger has nothing to do with the software's reliability, no?