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.

As Firefox will introduce Manifest V3 which will make ad-blockers unusable, I hope they will not implement that as well … But since this is so new, this will not have any add-ons at all for the foreseeable future