add firefox extension template

externs/webextension_polyfill/tabs/MutedInfoReason.hx

@@ -0,0 +1,10 @@
+package webextension_polyfill.tabs;
+
+/**
+	An event that caused a muted state change.
+	
+	"user": A user input action has set/overridden the muted state.
+	"capture": Tab capture started, forcing a muted state change.
+	"extension": An extension, identified by the extensionId field, set the muted state.
+**/
+typedef MutedInfoReason = String;