add firefox extension template

externs/webextension_polyfill/tabs/MutedInfo.hx

@@ -0,0 +1,25 @@
+package webextension_polyfill.tabs;
+
+/**
+	Tab muted state and the reason for the last state change.
+**/
+typedef MutedInfo = {
+	/**
+		Whether the tab is prevented from playing sound (but hasn't necessarily recently produced sound).
+		Equivalent to whether the muted audio indicator is showing.
+	**/
+	var muted : Bool;
+	/**
+		The reason the tab was muted or unmuted. Not set if the tab's mute state has never been changed.
+		Optional.
+	**/
+	@:optional
+	var reason : MutedInfoReason;
+	/**
+		The ID of the extension that changed the muted state. Not set if an extension was not the reason the muted state last
+		changed.
+		Optional.
+	**/
+	@:optional
+	var extensionId : String;
+};