I'd suggest asking on the McAfee support site: https://community.mcafee.com/t5/True-Key/bd-p/truekey

McAfee have been trying to fix the problem, but have now referred me to my ISP and to Firefox. As far as McAfee are concerned their software is installed correctly and running correctly. They say the issue is with the ISP or Firefox. My ISP is BT who are also scratching their heads, or was it was their a***'s, either way they cannot find the problem at their end. So sorry guys but over to you.

True Key is developed by McAfee, which is not affiliated with Mozilla (the makers of Firefox). While third parties such as McAfee can make extensions that run inside Firefox, Firefox is not responsible for making those add-ons work.

I'd actually strongly suggest you uninstall McAfee entirely, it's actually not a great anti-virus and Windows Defender (which is built into your computer) is far better. Then run a monthly Malwarebytes scan (www.malwarebytes.com)