Totally fair. Also you can't resize the window on windows! Oof. I normally use this extension on Linux where it works for my purposes.

https://addons.mozilla.org/en-US/firefox/addon/pwas-for-firefox/ might work for you, but requires an external program to manage separate user profiles and/or firefox installations.

I think the best hope is for official support, which looks like it might be coming soon... see https://connect.mozilla.org/t5/discussions/how-can-firefox-create-the-best-support-for-web-apps-on-the/m-p/85327/highlight/true#M32616