The minimum browser versions have been updated to Chromium v80 and
Firefox v78.
Chromium v80 is >3 years old, and very few users use older versions
than that.
Firefox v78 is two extended support releases ago (v91 and v102 ar the
previous and current ESR releases).
Resolve an inconsistency between Chrome and Firefox with how the
contentscript runs in an iframe.
This should have no user-facing impact, it's just meant as a safeguard
in case something unintentionally gets included in the contentscript.
The web accessible resources were removed in #14834, but they were
accidentally preserved in the v3 manifest. We no longer need any web
accessible resources, and leaving these in the manifest exposes us to
potential security issues.
