View source for Feed Handling/Default Feed Reader

= Background =

Applications register themselves as the "default" feed reader by claiming the "feed" protocol in the system's protocol registry. 

In general, Firefox should not touch this setting, '''unless''':

* An automatic action is configured for clicking on feeds AND:
* That automatic action is to subscribe in either a web reader OR Live Bookmarks

e.g. consider the following scenarios:

== Scenario 1 ==

User visits a feed document and is shown the pretty print page and presented with the choice to subscribe. They subscribe with a reader, but do not select the "never ask again" checkbox. 

'''feed: protocol system handler:''' claimed by Firefox.

== Scenario 2 ==

User visits a feed document and is shown the pretty print page and presented with the choice to subscribe. They subscribe with a desktop application reader, and select the "never ask again" checkbox.

'''feed: protocol system handler:''' set to desktop application.

== Scenario 3 ==

User visits a feed document and is shown the pretty print page and presented with the choice to subscribe. They susbcribe with Bloglines or Live Bookmarks, and select the "never ask again" checkbox.

'''feed: protocol system handler:''' claimed by Firefox.

== Other Application Case Studies ==

User is sent a link via AIM to feed content, e.g.:

http://www.foo.com/index.rdf
feed://www.foo.com/atom.xml
etc...

If they always use their desktop application reader to open feeds, and Firefox is configured not ask every time, then clicking either of these links should take the user straight to that application. 

If the user is asked every time, or a web handler or Live Bookmarks are used as the automatic action, Firefox should open either of these links and either present the choice, or subscribe with the web reader or Live Bookmarks.

This makes sure that just because the user once installed a feed reader, they still have the opportunity to subscribe to links in their favorite web reader, if applicable, even if they stop using the desktop application feed reader. 

This works because Firefox handles http:// protocol URIs typically when set as default browser, and should claim feed:// according to the heuristics above.