add_resource.js

function createResource(resourceName, uriPath) {
    let resource = Services.io.getProtocolHandler("resource").QueryInterface(Ci.nsIResProtocolHandler);
    var fileuri = Services.io.newURI(uriPath, null, null);
    resource.setSubstitution(resourceName, fileuri);
}

// in startup()
createResource('toolbar-buttons', 'chrome://toolbar-buttons/content/resources/');

// in shutdown()
let resource = Services.io.getProtocolHandler("resource").QueryInterface(Ci.nsIResProtocolHandler);
resource.setSubstitution("toolbar-buttons", null);