That probably won't work. The add-on reviewers are very strict about running arbitrary code loaded from somewhere else, and no add-on using it would be approved

That probably won't work. The add-on reviewers are very strict about running arbitrary code loaded from somewhere else, and no add-on using it would be approved. (After all, you could change your code and store people's emails.) That being said, you should be able to use this to load new selector strings for the API: https://addons.mozilla.org/en-US/developers/docs/sdk/latest/modules/sdk/request.html https://addons.mozilla.org/en-US/developers/docs/sdk/latest/... http://developer.chrome.com/extensions/webRequest.html http://developer.chrome.com/extensions/webRequest.html