It's not as clear cut as that. Someone actually has to bring the antitrust suit. In the last few decades, the US government hasn't been doing as much of that as they used to. Once in the suit, then you have to argue about the "relevant market" to determine which other companies count as competitors. This question probably consumes more time in an antitrust case than any other. Then you have to argue about how much of that market the defendant controls.

At least in the US, I know of no legislation forbidding a dominant OS maker from preferring some browser. Microsoft was put under some rules like that after their antitrust suit, but that's very different from legislation forbidding all companies from having such preferences.

Europe is different. I've not studied their antitrust law as I have that of the US, but I believe they're more aggressive on both the enforcement and legislation sides. I think the average American would be better off if US antitrust policy were more aggressive than it is now.