Close TB, open the profile folder, which is located by default at:

C:\Users\username\AppData\Roaming\Thunderbird\Profiles\xxxxxxxx.default

AppData is hidden by default. Open prefs.js in a text editor and change the line with layout.css.devPixelsPerPx so the value is -1.0, save the js file, restart TB.

Thanks for your prompt reply. I have tried to open the Profile folder using the address as stated with my username but the message I get from File Explorer is 'Windows can't find ...check the spelling and try again' Any ideas please? Done this several times and checked.

The xxxxxxxx.default is just a placeholder name. The x are a random string of characters. Open this location:

C:\Users\username\AppData\Roaming\Thunderbird\Profiles\

and then open the relevant profile subfolder.

If you can open TB, it's also accessible from Help/Troubleshooting, Profile Folder, Open Folder.

I opened C:\Users\username\AppData\Roaming\Thunderbird\Profiles\ and then opened the file a5yktgke.default. I couldn't then find pref.js although lots of numbered prefs files. What is the relevant profile subfolder.? I was expecting to see layout.css.devPixelsPerPx to change the values. Thanks for your patience.

The relevant profile folder is whatever profile is opened when TB is launched. That may be different from a5yktgke.default if there are multiple subfolders of Profiles. If you changed layout.css.devPixelsPerPx to anything but the default, it will appear in prefs.js (see picture), and prefs.js is present in every profile folder.

The only Profile folder that I have is a5yktgke.default. This folder has many files but does not have a prefs.js. I could not find any multiple subfolders of Profiles. I am an average user and I feel a bit of my comfort zone but I want to resolve this problem. Thanks

Post a screenshot of the profile folder which includes the files beginning with p, as in the attached picture. On the View tab of File Explorer, check the box for Hidden items.

Please see screenshots of the profile folder as pictures below, which includes all the files beginning with p. No hidden items found on the view tab of File Explorer. Thanks

Your picture shows the file as 'prefs' because you have hidden extensions for known file types in File Explorer Folder Options (see picture). Uncheck the box and you will see prefs.js.

I have followed the instructions and now have the same view as above. I also have prefs.js in the long file list. Where do I go from here?

Follow the instructions above: https://support.mozilla.org/en-US/questions/1336435#answer-1413416

I have followed the string above. I then opened prefs-1.js This is a very large file with many lines but could not find "layout.css.devPixelsPerPx" Also there was a note "Do not edit this file".

Open prefs.js, where you will find the the relevant string. Use the search function in your text editor.

Success at last. Many thanks for your help, it has been a challenge!