I'd like to see a more robust feed API in Firefox. You guys already have a very competent system for subscribing and managing feeds. However, there is no real API to build "feed applications" on top of.
It would be awesome if Firefox internally handled the file store for downloading and parsing feeds, then exposing them through a clean, simple, and uniform API. We can then focus efforts on managing stuff like whether a feed has been updated since last viewed in one core place, instead of pushing complex code up to the view (extension) level.
I've got a few cosmetic ideas... at the moment, the Subscription button in the address bar is static, there is no UI response when the mouse passes over it. Perhaps an effect could be added to indicate that this is a button, perhaps something similar to the effect on the cross button to close a tab.
The other thing I have found annoying, there doesn't seem to be an easy way to determine the address of a feed from the Subscription icon... it just gives you the name of the RSS feed. It would be good to perhaps be able to right click on the Subscription icon to "Copy Feed Location" or something similar... similar to the way you can right click on a link and "Copy Link Location"
Rather than open in an external reader, can't we straight integrate RSS with Firefox?
Take a look at the Sage extension, it renders a pretty Web page when displaying an RSS feed. Currently, if you hit say http://www.slashdot.org/index.rss Gecko spews out an XML tree and complains about not having a DTD or stylesheet. One possible consideration for Firefox 2.0 would be to adopt the strategy Sage uses, and directly handle feeds that don't bring a DTD by rendering a page from our own DTD.
To carry this out, Firefox would have to supply a chrome:// that aggregated a list of feeds. This would have to be prohibited from being accessed by any Web page on the Internet, as it would be an information leak if they could read this information (exposes user's RSS feeds, thus user's interests). Additionally, visiting an RSS feed directly should aggregate the RSS feed without opening a whole RSS reader; there should be a button in this mode to add the feed to your list of feeds.
Some integration with Firefox's UI would be important too. There needs to be an uninvasive feedback mechanism to inform users that they have new RSS feeds. Perhaps the orange RSS LiveBookmark icon could appear, and slowly pulse (not flash; just fade to translucent and back over several seconds). This would of course only be useful given a mechanism to periodically update feeds.
The other idea would be to just use something similar to Sage; but this adds a sidebar. The sidebar eats screen real-estate, and is undesirable.