The &quot;Default Browser&quot; plug-in is not responsible for the notification mentioned in that tweet. The plug-in has no opportunity to execute code when Firefox is launched, only when instantiated by a web page.<p>Besides, there&#x27;s no need for a browser plug-in to detect when Firefox is run: the LaunchServices framework responsible for launching applications on OS X can simply check whether the application it is launching appears to be a web browser.