AFAIK FF is implementing Manifest V3 so that those add on developers who are migrating to it don’t lose FF compatibility. As long as they don’t deprecate Manifest V2 for those that need it, ad blockers will continue to be usable.
edit: although I’ll add that there’s a major problem here that Mozilla simply can’t address. If ad block stops working on chromium browsers, and ad block users all migrate to Firefox, then that makes it a lot easier for Google or others to target users of those browsers and deny them access to their sites. Imagine if Google goes scorched earth on all browsers that support manifest v2.