User Agent: Mozilla/5.0 (Windows NT 6.1; rv:58.0) Gecko/20100101 Firefox/58.0 Build ID: 20180206200532 Steps to reproduce: Installed the RSS reader add-on, 'Brief'. Verified it registered as an RSS reader, as it was listed as a subscription option. Note: checking the 'always use' option has no effect, new subscriptions always go to the prompt page with 'Live Bookmarks' set as the default (see separate bug https://bugzilla.mozilla.org/show_bug.cgi?id=1426290). Went to Options → Applications and selected one of the RSS items (Podcast, Web Feeds, etc.) Clicked the 'Action' drop-down. Actual results: 'Brief' is not listed as a handler for the type (see attached image). Selecting 'Use Other' only lists external applications. Expected results: 'Brief' should be listed as a handler for the type.