Doesn't Android open the Chrome web view too? On F-Droid I think you can get a Bromite web view, but I'm not sure how these web views work, and if I can get a Firefox one.
When you open a link in an app, you're often directed to a Custom Tab (you can recognise them by the menu in the upper right corner being the browser's). Those are handled by your default browser. I'm using Firefox Focus in that capacity and it works great.
