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.
They won’t care, they have the consumerist crowd locked down. The crowd that buys dozens of Stanley Tumblers so they have one that matches any outfit. There might be more of us who care than there used to be, but the average iPhone buyer doesn’t care and Apple knows it.