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.
But the privacy is just a facade, right? Like with that recent scandal about the government requesting push notification info, Google of all companies was actually only handing it over with valid warrants while Apple was giving it to any law enforcement who asked.