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.
Because then Apple fanbois will complain and tarnish Mozilla’s/Firefox’s reputation
They already do that, they can say whatever, they will be isolated with their shitty Apple products and if they want decent browsers then they will need to use decent systems. Apple can’t abuse of their power and force us to follow their abusive rules, they can’t even have a decent UI desktop. They are so bad programmers.
Developing a crippled port that is limited/restricted by design due to Apple policies would not really help Mozilla’s/Firefox’s reputation anyway. Apple fanbois will complain ether way.
If those fanbois want a Firefox app on Apple systems, it’s Apple the one they should complain to.
Or walk away from.