Thanks, it works! (I had to access about:config via chrome://geckoview/content/config.xhtml)

While you're there flip general.aboutConfig.enabled to true so you won't have to use the silly config.xhtml URL again.

What a great thread!

Thank you and previous poster for sharing how to get that working.

That setting unfortunately resets after restarting the browser. But the width setting stays.

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!

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).