If you are like me and just want a SIMPLE user agent switcher for a few sites, this is the one. Just go to the site you want to use, click on addon icon, "override for domain", select a link variation and you are done. I use it mainly on instagram desktop and I have tried 2 different similar addons before and they either didnt work, or were too complicated. I figured this one out in 2 seconds literally.

Edit: recently this addon began breaking outlook and twitch pages for me. This happens when its not set to "Default", but instead I set it to what I use "Windows / Firefox 83", and it broke many things on those pages to a point I was using a different browser. Just set it to "Default" when not in need of its services.