As I sit in front of Mac desktop computer most of the time, I rarely use Facebook Messenger or Apple iMessage Apps on a phone, preferring to have them open in the Dock, this has worked well for me for years. Microsoft in their wisdom (again!) decided to drop the Messaging Desktop App on Mac so it no longer works. If I get a Messenger notification on my phone or watch, I have to open Vivaldi, my browser of choice, open Facebook and then Messenger to reply.
Not any more....A web search bought up a solution. Open Safari on my desktop, go to Messenger.com and login with my FB account. Then, go to the 'Share' button (top right with an upward pointing arrow), click 'Add to Dock', I now have an 'App icon' for Messenger in my dock I can just click to open Messenger instantly on my Desktop, exactly as I used to have with the Messenger App for Mac.