Did you try the method suggested here?

https://support.mozilla.org/en-US/questions/1493765#answer-1712770

sfhowes, thanks for responding! What I found in the above link was:

Open Thunderbird, Go to Settings-General; at the bottom of that section you'll find a button for Config editor; click on that and search for this line: widget.non-native-theme.scrollbar.style; click on the edit button on the far right of that line and change the 0 to something like 28 and hit the enter key on your keyboard. The change should become instantaneous, but you may need to close and re-open Thunderbird. 28 seems to be a fairly decent size, but you can adjust it as necessary to suit your needs.

I tried that, but it had absolutely no effect on my scrollbar width, even with a setting of 50 instead of the 28 referenced. I found another reference to making the scrollbars always visible, which I also found and checked, and then restarted Thunderbird. But the changes did not take effect. Is there another setting that would be effective?

Thanks again.

The preference widget.non-native-theme.scrollbar.style takes the value 0 - 5, and choosing 4 is the widest, at least here in W11.

https://www.reddit.com/r/Thunderbird/comments/1fz9f5j/comment/lr02b8z/

https://support.mozilla.org/gl/questions/1503077#answer-1722732

I don't have Linux to test, so try that and report on the results.

The colours can be set with this code in userChrome.css and userContent.css:

:root {scrollbar-color: orange denim !important;}

sfhowes, that worked, thank you very much! I did try that number at first, but for some reason it did not work then. But this width is a big help.