I think adding it to the "common" bundle works, but we need to be sure to add it to other bundles that replace the common bundle - like https://github.com/nburka/bedrock/blob/master/bedrock/firefox/templates/firefox/base-resp.html#L89.

Doing a global search for `{% block site_js %}` showed "common" replaced with other bundles in about 9 templates. Need to make sure mozilla-image-helper.js is added to those bundles as well (where necessary).