How can I stop pop up notices from Mozilla saying that my McAffee licence has expired. I now use MS Defender

I terminated my McAfee subscription, as Windows 11 and Microsoft Office 360 have Microsoft Defender included. I keep getting Mozilla Firefox warning notices saying my McAfee licence has expired, Trojan Virus found on hard drive, multiple virus on my computer, Critical errors etc. I have had MS Defender do several scans and no problems have been detected. Can you help correct this or do I have a virus that Defender cant detect? I have tried to copy one of the notices but it comes up as a link to further notices and I am concerned it is another virus. The notices are all Mozilla badged with suggestions such as Scan and fix, Turn on protection, Update windows and more. I am able to delete them, but after a few minutes they return. Regards Robert.