What is your anti-virus software?

Try to start Windows in safe mode with networking enabled.
Does the problem go away?

Hi Christ1. Many thanks for your help, much appreciated. Have tried safe mode, opened T'Bird and no popstate, but not connected to internet. All this is a bit beyond me! Am using as anti virus, Kapersky, in the back of my mind there is a thought that there could be a problem, what do you think. Could try and uninstall it again, do you agree? Again many thanks

... not connected to internet.

Did you start Windows in safe mode with networking enabled? Safe mode can affect WLAN connections, so you may have to use a wired connection.

Your problem possibly results from anti-virus blocking files in the Thunderbird profile. Windows safe mode disables anti-virus software. So safe mode is not the solution, but just a troubleshooting measure to verify whether anti-virus software is the culprit.

These are some general recommendations to avoid problems with anti-virus software.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it.

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Trying at the moment to uninstall Kapersky, not having much luck!

Have uninstalled Kapersky, seems ok, no popstate! Many thanks for your help Best wishes Eric

When your problem is fixed, can you mark the topic as 'Solved' please?
Thank you.

Thanks for all your help, appreciated

I have tried EVERY GMAIL CONFIG on the internet and I STILL CANNOT ADD GMAIL to Thunderbird. Any suggestions would be greatly appreciated.