(Note, this is intended as self-answer Q&A for a problem I encountered)
After trying to submit a WebExtension to Mozilla Add-Ons for public listing, the submission was rejected by a reviewer with the following message:
Your add-on includes a JavaScript library file that doesn't match any versions known to us. We require all add-ons to use unmodified release versions.
We accept JQuery/JQuery-UI libraries downloaded from 'ajax.googleapis.com', 'jquery.com' or 'jqueryui.com'; and used without any modification (downloaded and not copy pasted). Minified versions are better. (file-name change does not matter)
I'm sorry, but we cannot accept modified, re-configured or customized libraries.
Another symptom of the same problem: several "Unsafe assignment to innerHTML" warnings on initial validation coming from jQuery.
However, the file in question was indeed downloaded as a release, minified version from jquery.com and wasn't intentionally modified. So what happened?