Click on the divider and drag it with your mouse. It works for me on windows 11

Many thanks for your reply. I could make my folder pane wider according to the way you wrote, and is now able to use my TB mail far better than before. But the width of the pane is not good enough for me to see full folder names. In case I choose "Classical Layout" , divider drag works very well to show full folder name even if it is long name. Question is only in the case I choose the "Vertical Layout".

Accordingly, there might be, I guess, a special way in the "Thunderbird's Setup Editors Options" which amateur users should not easily change system logic value.

Thank you Matt, again.