The string in the allowed_extensions
array must match the ID you give your extension, which you have to do manually at least while your extension is not signed. If you don’t explicitly define an ID you will get one assigned once the extension is signed and would have to then adjust the manifest for the pkcs11 module. See https://developer.mozilla.org/en-US/Add-ons/WebExtensions/manifest.json/applications for documentation on how to set an ID in your manifest.json of the extension.
Ensure both manifests use the same ID.
P.S.: you can use ``` to start and end code blocks (like on GitHub), so multi line code gets easier to read and indentation is preserved.