The files will then be available using a URL like: "moz-extension://<extension-id>/<path/to/resource>".

For example, consider an entry like this:

"web_accessible_resources": ["images/my-image.png"]

If your extension's ID is 944cfddf-7a95-3c47-bd9a-663b3ce8d699, then this resource will be available at the following URL:

moz-extension://944cfddf-7a95-3c47-bd9a-663b3ce8d699/images/my-image.png