Error uploading a manifest v. 3 extension to the Mozilla add-ons store

The extension runs fine on the desktop Firefox browser, but the upload page rejects it.

Did you zipped folder content or whole folder?
You need to zip only files inside the folder (where the manifest.json file is).

D’Oh! That was my mistake: I zipped the folder. When I zipped the contents, I got another error and set of warnings. Am looking at that now.