I do a lot of debug between windows and android versions of Firefox. In windows I'm able to use an extension called "toggle dark mode" by Cimbali. No matter what I try this won't install on mobile. Lots of my tests involves switching modes to ensure that my css for both media queries looks good.
I would like to propose a submenu in the hamburger menu to quickly switch between dark/light/device theme. Currently you have to click on the hamburger menu, scroll down to settings, find "customize" then make your change. This is quite obnoxious when you need to switch back and forth quite often.