Port to ExtensionSettings policy for PBI

Thomas Duckworthrequested to merge
work/tduck/plasma-integration-ff into master

OpenQA found a bug! With the latest version of Firefox, our current extensions policy leaves Plasma Browser Integration disabled. Use the better-supported ExtensionSettings instead, which allows us to enable it. See https://firefox-admin-docs.mozilla.org/reference/policies/extensionsettings/

Merge request reports