Lets hope they stay true to their words and do not deprecate Manifest V2 later, since firefox is an open source project, theoretically anyone could fork it and build this on their own, but I heard compile-times for firefox is long. And as complexity of the web increases maintaining your own forked web browser will become harder and harder. That is why projects like Ladybird are important imo.
As more and more webpages do not support firefox anymore (Notion did not work for me today) the web will become unusable in a dystopian Manifest-V3 only future.