Add-on FooBar is broken!

That's probably expected.

Our add-on No Resource URI Leak's purpose is to block any potential privacy leak via resource:// URIs. (Also chrome:// URIs) Since Firefox still doesn't provide a way for an add-on to include resources in tabs without exposing them to the websites, many add-ons (often just to create a small button) use exposed files to do this. Exposing files to random webpages is a privacy leak. It will reveal your browser's precise configuration (OS, add-ons, exact version, etc.), enabling pretty good fingerprinting.

Whitelisting a particular add-on is not a solution. If you a developer, please see [Proposal] Isolated content environment and How to determine the MIME type of the loading document in a Content Policy instead.