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.
You can pay a lifetime licence for 10$ https://apps.apple.com/fr/app/adguard-pro-adblock/id1126386264?l=en-GB
I ended up getting it from them directly. I am on Android 90% of the time and don’t pay for the license from the Apple store. No idea if that one is a single device or the same as the multi-device. I tend to not do things from the Apple store as I don’t have an iPhone and they seem to want or assume you have one. Just like I can never use their watch (not that I want one).