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.
I really hope the EU gives them the middle finger and tells them to apply the law in good faith, not like this.
Still can’t do much for markets outside the EU though. Countries like the US, Korea, Japan, Taiwan, Australia should implement similar laws. That would force Apple’s hand.