Just a guess but Avast is immediately suspect here.

To diagnose problems with Thunderbird, try the following:

• Restart Thunderbird in troubleshoot mode (Thunderbird Safe Mode historically). On the Help menu, click on "Troubleshoot mode" and click to continue into that mode when prompted. If Thunderbird works like normal, there is most likely an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click here for instructions on starting windows 10 in safe mode

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Note the operating system mode requires you to use a wired internet connection. A less effective method of disabling all startup apps (Search startup app in the windows 10 task bar) is to disable all startup applications. This includes security software.

Hey Matt appreciate you taking the time to respond with such a comprehensive reply, however not being very tech minded I am a bit daunted reading yr instructions. Also I have windows 11 not 10 not sure if that makes a difference - however I have found Avast very helpful in the past so I think I'll contact them to see if they can help. Previously Avast solved another issue with a remote access to my laptop so that may be the way to go. Cheers Alex

Interesting that the copy of Edge you used to post on the site reports you as using Windows 10. Not that Windows 10 and 11 differ much. Except for a new coat of paint not much really changed under the hood.