That's not how anti-trust works. Just because a player does not hold as much of a market they used to, does not mean they do not have a monopoly or cannot abuse it.
Microsoft absolutely has a monopoly on the desktop OS market. And even without that monopoly this is abuse. With monopoly, this becomes abuse of their power in addition to being abuse of their users.
For example, the consumer choice feedback loop breaks down when scale gets large enough that market signals from individual decisions are smaller than the amplitude of the noise, making it easy to neutralize them with marketing. We are heading in a direction where market breakdown occurs long before monopoly power, and cartel-like behavior is more likely. The largest players are so large they have more incentive to cooperate to keep everyone else down, while maintaining only nominal competition with each other.
That doesn't mean anti-trust is no longer relevant, but that people arguing for it need to revise their position. Antitrust has never been about monopolies, but about abuse of power in the market. It was only a historical coincidence that that lined up with monopolies when the total economy was smaller. Corporations have now become so large that market failure begins long before monopolies, and our trust-busting strategies need to adapt to that.
Maybe it's possible to run Firefox through the new Linux apps option they added?
Microsoft has fallen in the market-share of OSs, can no longer be considered monopolist move.
But yes, it's a new low, there was never a "new Microsoft" just better PR.