Incompatibility with Alternate Player for Twitch.tv

[RickyRomero]

The addon Alternate Player for Twitch.tv embeds Twitch streams in a webview bundled inside it. This webview also embeds the Twitch stream's chat in an iframe, which Shut Up is allowed to block.

However, because Shut Up can't control pages featuring moz-extension:// URLs, it can't control its own blocking of this iframe, leading to a situation where the chat is blocked but can't be shown with the toolbar button (which is disabled).

This is a very strange edge case and I'm not yet sure if it's straightforward to fix.