You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

Cdn.doubleverify.com is a suspicious website that may contain ads or pop-ups which annoy many computer users. It can take over web browsers like Internet Explorer, Mozilla Firefox and Google Chrome and searching facilities. If you have no idea how to deal with this issue, just follow the instructions here to get rid of the malware step by step.