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.
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.