Well, it's fairly simple, really: you want to access your favorite social network and it doesn't allow you to do something at you desktop. It's really that ridiculous: the fully fledged computer at your home can't certain things only allowed to your goddamit mobile phone with a processing power worth a potato. No problem, we got you covered.
On Firefox, if you want, you can hit “Ctrl + Shift + M” (Chromium and Chrome opens the stupid login box with this shortcut) and you go directly to responsive layout mode. Be sure to select a device (go to the “Responsive” dropdown and select, let's say, iPhone 7). You should get something like this: