I guess I made the mistake of developing for Firefox first with their documentation/guides. I am using rollup and the polyfill plugin per some suggestions, but it certainly isn’t generating anything compatible for v3, and I believe it breaks on the background script vs. worker thing.
Up until now I've just been publishing the exact same extension for Firefox/Chrome/Edge (and also as a user script), as Firefox supports using the global `chrome` object for everything I've needed it for. The only browser-specific stuff has been targeting different browsers with CSS in the options page to make it blend in with the default light and dark mode themes in Firefox/Chrome/Edge.
Looks like I'm in the same boat the next time I need a new extension.
Only good thing about this is that Mozilla is working on v3 support.
> Manifest v3 is a large platform project, and some parts of it will take longer than others to implement. As of this writing, we are hoping to complete enough work on this project to support developer testing in Q4 2021 and *start accepting v3 submissions in early 2022.* This schedule may be pushed back or delayed due to unforeseeable circumstances.
Unfortunately, this article was written in May 2021. As far as I can see there's no update to this article yet, so we don't know what "early" 2022 means.
> https://blog.mozilla.org/addons/2021/05/27/manifest-v3-updat...
Source: email dated 2021-12-12
https://developer.chrome.com/docs/extensions/mv3/mv2-sunset/