To be fair, Apple is a bit hostile towards open source developers. I can write windows and linux software from both windows and linux, but to write apple software I need a mac and a developer account that costs money. If firefox depends on patches from the community, they will receive less patches for macos features because of this asymmetry.
Firefox may be open source, but Mozilla is a company with a paid development team. It is exactly these types of issues - UX, UI - on which the dev team should be working. I'm sure that a lot of features come from the open source community, but the polish and sound implementation is on the dev team.
