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.
People should fully own the computers they buy, regardless of which company they buy from.
This means root access and a replacable primary bootloader, let alone just being able to install apps not on a curated market (what Apple calls sideloading). macOS and Windows both manage to allow root access, and so do certain Android devices (and obviously other OSs as well). Replacable primary bootloaders are more rare, though, especially in ARM devices due to efuse-based secure boot in the CPU that is impossible to turn off. There’s only one phone I can think of that allows for replacing the primary bootloader (Shift 6mq).
We shouldn’t allow for artificial restrictions placed by corporations on devices they sell, because as we have seen time and time again, companies copy each others’ restrictions, especially Apple. Same goes with game consoles, IoT devices, Smart TVs, etc. And before you mention the potential for piracy, DRM is an artificial restriction placed by corporations, and should also be removed from devices.
Anything less means that you don’t own the device that you paid for.
Apple is clearly attempting to comply with the EU DMA in bad faith so that they can maintain as much control over their users and app developers as possible.