No, for certain changes in Manifest V3 there is no way to polyfill the V2 API because the functionality is essentially removed/impossible to do now.

Also, other aspects (like switching from background pages to service workers) require a change in programming model (moving from simple variables to message passing/persistent storage) as your service worker can be killed off at any time.

> No, for certain changes in Manifest V3 there is no way to polyfill the V2 API because the functionality is essentially removed/impossible to do now.

But isn't the whole announcement about Firefox also going to support v3 (e.g. you only need to write for v3) except that it still allows some of the removed functionality _additionally_ to the v3 APIs Chrome has?

> Also, other aspects [..]

Yes, that will be a major change. But as far as I understand one you have to go through anyway if you want to support Chrome.

> But isn't the whole announcement about Firefox also going to support v3 (e.g. you only need to write for v3) except that it still allows some of the removed functionality _additionally_ to the v3 APIs Chrome has?

Right - maybe I misunderstood the previous commenter's question, but I thought they were asking if it would be possible to make a translation layer that would allow developers to seamlessly support Manifest V3 without changing their Manifest V2 code.

What you are saying is correct though - Manifest V3 code will be universally supported, with a few browsers (Firefox, and Brave I believe) also supporting some of the V2 APIs on top of that. But if you use those APIs it won't work in Chrome.