User Agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:56.0) Gecko/20100101 Firefox/56.0 Build ID: 20170926190823 Steps to reproduce: As a WebExtension developer, I want to be able to programmatically add a new ContentHandler so that the user is able to subscribe to a RSS/ATOM feed by using the standard dropdown menu next to "Subscribe to this feed using" with my extension as one option in the menu. If there was an API (can be and probably should be limited so that only WebExtensions can use it) to add a contentHandler, users wouldn't have to do the manual rather technical tinkering in about:config. See https://www.reddit.com/r/firefox/comments/llpvg/how_do_i_subscribe_to_rss_feeds_to_google_reader/c2tpcr3/