Firefox is now at version 12.0. Basic AppleScript support (such as getting the URL of the current window) hasn’t worked since version 3.5, and it was intermittently broken in some earlier versions.
-- Michael Tsai
Realistically, Shelf is going to stop being useful at some point in the next couple of years.
The Apple App Store doesn't permit the sort of AppleScript usage that it relies on, so it's never going to be an App Store app.
As a result (and also as part of a longer-term trend, really), no-one really cares about AppleScript any more. Not that Firefox was ever an exemplary citizen in this respect.
Google are killing the social graph API that provided most of the discovery for it.
Increasingly few of the services that I'm interested in provide RSS feeds to their data, preferring to be iPhone apps or walled gardens.