https://commission.europa.eu/strategy-and-policy/priorities-...
The EU's position is that a company is large and influential enough, with an entrenched market position, it does not get to "do whatever they want" because of the harm that causes to the economy overall. A monopoly is not a criteria for this.
I'm not sure I understand: this would be applicable if Apple was the only provider of phone/mobile OS. But... Apple is just one of the players; if I was adamant that Firefox - which, again, I've been using for the past 20 years - is the only browser I want to use on mobile, I'd simply give up on iPhones and buy something else. So, again, as long as I have a choice, where is the problem?
Btw, Apple is not "just one of the players". Them and Google completely control the mobile computing software market. Both them and Google are now required to stop abusing their position.
In your example, Firefox/Mozilla is harmed, because they're locked out of the iOS market unfairly.