gp @Vlad From a user side of view, these kind of feature would be useful for people who use public wifi frequently --- I believe they are not minority.
Usually when you connect to a wifi that requires login, a independent login window will pop out and nothing in browsers is needed, but this is buggy on macOS at least from my own experience. The popup windows frequently just don't show up (I would say 3 out of 10, higher when something like VPN involved). When this happen there are two cases, either the wifi just don't work at all, or you can open any page in the browser and it will be redirected to the login page, if some pages already in your browser are loading, they will all be redirected.
The Firefox feature in my screenshot solved the second case. You can just click the button to open the login window without interfering any current page. It might be messy technically, for asking browser to do something that is not supposed to be its job, but it's pretty handy on the users' side.