[Lamprey]

Alternatively, you can download Firefox Nightly instead of regular.

"about:config" just works in Nightly. No fuss.

You can sideload extensions in Nightly, too, after you activate the developer options. I don't think they've added that to regular, as yet? At least not with as much flexibility.

Anyway, I'm gonna try this mobile desktop mode thing and see how it goes. Thank you to everyone!

[moho]

Installing extensions from file is available on the release build as well, after enabling dev options.

I think the only difference is nightly allows installing unsigned extensions, which I don't personally have a need for (as getting a personal/non-published extension signed is very easy).