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.
And that’s coming from someone who has “applelover” in their username. They fucked up big time by pissing off even their most loyal fans.
(/j)
Honestly, I don’t know how many normies this will upset. Pretty much only techies follow this and the people being annoyed by it might just blame the app developer and not Apple. Time will tell