Flight Scan - Google Flights Tool

Click on the icon and you will be presented with travel aggregate websites that you can toggle on or off, choosing whether to see a flight comparison from that site or not.

When you go to Google Flights, find a flight and click on that particular flight, a popup window at the top right of your browser will appear with a button that says "Compare". If you click this button, it will create a new tab for each site you have toggled ON and it will automatically search each site for the same flight you have up on Google Flights.