Hi andrew.t.pearson, glad to see you on Mozilla Support Forum.

First remove the current Firefox Quantum from your system completely.

Then, Downlaoad a fresh copy and Install.

https://blog.mozilla.org/blog/2017/11/14/introducing-firefox-quantum/

Hope it Helps ! Thank You.

Thanks Shashank I have uninstalled it, downloaded the latest installer exe and the latest version is installed. I use it for a while but say on a restart I have Quantum back. There is something somewhere on the PC which is re-installing Quantum. Do I have to uninstalland remove all FF folders and profiles before installing the latest version again? Cheers Andrew

Hello Andrew, Try this one I have also a problm with quantum. My fix was to do a System Restore back to a previous time, BEFORE Quantum was downloaded. I'm lucky that I do a Backup & Restore [Win 7, 32bit] EVERY week, so I just restored to my last back-up point.

Remember to turn OFF automatic Firefox updates, so you don't end up getting Quantum again.

Hi Himanshu Unfortunately 1909 installed recently so no restore point without Quantum already installed. Auto update is turned off already which doesn't stop Quantum overwriting latest version e.g. v 76 Cheers Andrew

This ended up being a Comodo problem. I uninstalled it a while ago, but the file remained and after I deleted it, it solved the problem:

C:\Windows\system32\iseGuard64.dll