I have virus and trojan horse pop up ads coming from Mozilla and haven't been able to delete them

I have several pop up ads stating viruses and trojan horses have been found. These are all trojan horses that they want to infect my system with. They are coming from Mozilla Firefox and I have no idea how to delete the pop up ads from my screen and computer. I do know they are false as I have McAfee and Malwarebytes and they both show that I do not have any viruses. I know if I ask them to fix this they will infect my computer. I need to know how to delete them thanks