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.
Basically, the person who marketed crocs and made them popular was hired to market Stanley tumblers, and they are genuinely one of the best at their job.
I don’t understand it, but they are very good at what they do