Recently “Search Enhance” ads started to appear on the results for my Google searches.   After a little work, I discovered that the cause was an add-in that was bundled with a Firefox add-on that I downloaded to capture YouTube videos.
To disable “Search Enhance” ads, you need to go to FireFox > Add Ons > Extensions and disable “Video File Download”, or my specific case “Video File Download – Download YouTube Videos 1.5”.  Once that is disabled, “Search Enhance” ads will no longer appear.