[edit: Just installed windows 10 today and I can still resize the window. The way I'm using this is to "copy web app link" then make a windows shortcut (right click, new, shortcut) and type in "C:\Program Files\Mozilla Firefox\firefox.exe" "moz-extension://bb731ad4-2288-4961-9b56-57cb731802d5/app/https%3A//addons.mozilla.org/en-US/firefox/addon/web-app-maker/reviews/" for the location of the item, where you put your link for the moz-extension part.]

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