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.
Apple does not care and will never care about open source other than the bits it has to care about because they’re a part of Darwin, their core.
They’re a company offering a particular “experience” and open source products do not fit into that model well at all. I use apple phones because I’m partially blind and for a very long time the accessibility story on Android was a screaming nightmare (I’m told it’s got better) but I have no illusions that they’re anything other than a profit seeking MegaCorp with all that implies.