A manifest tells the browser all sorts of information about an extension, from what its name and version number are, to what permissions it’ll use, and what versions of a browser it’ll run on. New versions of the Manifest file format used by the browsers change what features extensions have access to, and can mandate changes. For example Manifest V3 in Chrome no longer allows a developer to load code from a remote server.

https://www.theverge.com/2023/1/17/23559234/firefox-manifest-v3-content-ad-blocker