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.
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.