If it's simply occurring when you actually open a new tab, then the 'about:config' procedure, in Jno's link, will fix the problem.
However if a new tab is being opened without you calling for one, then it suggests that there's malware on your computer. Download, install and run the free version of Malwarebytes Anti0Malware to get rid of it:
https://www.malwarebytes.org/antimalware/
(When installing,take care to deselect the option to take a free trial of the 'Premium' version).