You enter thunderbird previous versions into a Google search and you get this site.

https://archive.mozilla.org/pub/thunderbird/releases/ At first glance it is a little daunting. First pick the version. Then pick your operating system. Windows is win32. Then a language. en-us is English USA. Then click the .exe file to install.

Nearly there! Thanks to your help I have installed the older Thunderbird to the new notebook, with all the extensions in my laptop system. Unfortunately, it says that Lightning is incompatible with TB 52.9.1, when I know I am running it on the laptop! Do I need an earlier version of Lightning, and where can I download it, if so?

Yes lightning and Thunderbird version have to match. Go to the add on tab in Thunderbird and click Extensions. Enter lightning in the search field and install it.

What a lovely fella! I had found and downloaded Lightning 5.4 but could not find any way to execute the file. Now all installed and working fine.

Many, many thanks

I know you got yours working but for others that might see this thread...

You do not execute Thunderbird add on files. You install them. If you find the add on using the search box as described above then it leads you through the process. If you download the file outside the add on tab do the following. Go to the add one tab in Thunderbird. Click Extensions. Click the gear icon and select Install From File. Navigate to where you saved the downloaded file and follow the prompts.