I wish Firefox can implement a similar behavior to Safari where
1. When Firefox is running in full screen, use a desktop UA
2. When Firefox is running in split screen/floating window, use a mobile UA
Currently it always uses a desktop UA, resulting in unreadable webpages when it is running in split screen/floating window modes.
I understand that it may be more of the website owners' fault that their terribly designed webpages (e.g. google.com) are taking the UA higher priority than the browser window width, but for now I think it's worth implementing the fix to improve the user experience for a bit. It may also be useful for the desktop and Android versions.