I regret you are having trouble. Thunderbird is totally free and no donation is mandatory. If you get a popup, asking for a donation, you can safely ignore it. Go to https://www.thunderbird.net/en-US/thunderbird/all/ and download the extended support release version.

Thanks david but I tried clicking https://www.thunderbird.net/en-GB/thunderbird/all/ (as I'm not in the USA) and the same thing happened. When I clicked Download I got the page where I have to say how much I'll donate. I closed this page and clicked the Download tab which took me to a page where I could click Download yet again. This took me to the donation page.

I've just tried the link. Once I've clicked on 'Download', it quickly plonks the installation file on the desktop and then proceeds to the donation page. There's an 'x' at the top right of that page (perhaps press F11 to toggle into full screen?) that removes the donation page - and then they thank me for not donating. The installation file is ready on the desktop to load.

Thanks very much for your help. Before downloading Mailbird I thought I'd go round the loop once more. The same happened except this time a window opened showing that downloading was taking place. TBird now seems to be flying as before - to my great relief.

I"m pleased all is working! And THANKS for your kind donation. :)