Mozilla is unhappy because the use of browser engines other than WebKit will be restricted to the EU, forcing them to develop two different apps.
For an independent browser like Firefox, managing two browsers is not easy, so it can be forgiven that this could be seen as almost harassment.
Also, the fact that the use of browser engines other than WebKit is limited to iOS means that the use of WebKit is still forced on iPadOS, which also increases the effort for Mozilla.
As someone who was an Android user and tried switching to an iPhone, I see lots of weird bugs and behaviors I never had with Android. Sure, the OS is slightly better, and rarely crashes, but everything else is a little bit worse.
Literally every Google app is better than Apple’s. Be it Google maps, keyboard, mail, calendar, etc. But because they aren’t made for iPhone, there’s all kinds of little bugs, mostly visual/UI related.
And then there’s all these nonsensical decisions that make my life harder. Hotspot can’t run with wifi on, you can’t record your calls (very useful when talking to banks, government, etc.), can’t even arrange the icons in “home” screen to fixed locations or make them smaller.
I finally figured out that when Apple fans talk about polish, they just mean it looks pretty and feels high-end. Which, sure, I can concede that. But that’s not what I actually care about.
Anyway, rant over, sorry to take it out on you!