Comment by NBPEL

It's called Sideload XPI, iirc Firefox Nightly allows you to sideload XPI if you enable Experimental by tapping Firefox logo in About 7 times.

But it's easier with Ice Raven or Smart Cookie Web Preview:

- https://old.reddit.com/r/browsers/comments/18ezd3g/iceraven_...

- https://github.com/CookieJarApps/SmartCookieWeb-Preview/rele...