It seems to be browser.viewport.desktopWidth; I found 500 to be a decent starting point on my phone.
Thank you and previous poster for sharing how to get that working.
"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!
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).