For FF and Chromium to, their source is open so if there ever was a need to make it fully European, it would be doable. Or did I miss something? (novice question, here)
in addition to the other reply, the teams working on browsers are massive. over 500 developers for each, and that's only the core teams without external contributors. you don't just put together a team like that in an afternoon.
The problem is that Librewolf, ungoogled chromium etc are soft forks, meaning they are completely dependent on the original projects. If for example Trump made a law banning releasing software as open source because that’s communism, Librewolf would likely cease to exist