I already do this. https://i.imgur.com/UwasqgD.png I thought it would be a more native experience to opening Messenger in the sidebar, but it isn't. One extra star because my Stylish theme worked, but that's not enough.
It works, but it loads Messenger very slowly on my (admittedly old) system. More importantly, it doesn't reformat Messenger to fit within the sidebar window, so left-right scrolling is required.
It places Facebook Messenger on your sidebar, unfortunately Facebook Messenger is not responsive design and that is Facebooks fault. Due to its use of React and Reason, it becomes really hard to inject CSS to make it responsive. Sorry but I can't fix messenger layout issues.
