Expert coming through
You are browser hijacked, it places ads everywhere even on your usually ad-free google.com
Revenue goes to the browser hijacker, not discoverygc.com or wherever the ad is placed
Burning firefox won't work, it'll also hijack internet explorer and google chrome and maybe even the opera browser you just mentioned
1:
Also, if you didn't realised yet or if noone mentioned yet there is a small text under the ads saying "ads not by this site" or smth different. ADS by adbest or smth like that. Then it's spyware on your PC creating these