This is a debugging tool, designed to assist developers in writing WebExtensions for Firefox. It tries to iterate through every API and see if it's an event that can be listened to. If it is, then it registers the event so that each time you hit the event it writes something to the console log. This is a really useful tool for seeing what events get called when.
To see the logs open the Browser Console: https://developer.mozilla.org/en-US/docs/Tools/Browser_Console
Icon is from https://www.iconfinder.com/icons/728899/alarm_alert_bell_event_schedule_timer_warning_icon#size=128 and is free for non-commercial use.