For apps that don't expose HTTP (a la nsIExternalProtocolService.isExposedProtocol), there needs to be a better story. Currently, this will probably just fail. The simplest thing to do would be to make nsIContentDispatchChooser not offer web-handlers at all in such apps. A better thing to do would be to recursively resolve the preferred HTTP handler and use that. Slightly simpler to implement would be to simply use the system default HTTP handler.

Firefox & SeaMonkey don't need this for 1.9, but other embedding (especially Thunderbird) are likely to be fairly annoyed if we don't get this. Nominating for blocking-1.9? Also note that if a "don't support web handlers in those apps" strategy is chosen for fixing this bug, we should also prevent register{Content,Protocol}Handler from popping up a dialog box in that case.