You can hide it by use of Custom CSS. I'm assuming that the right-most column you want to display is the date. However, it isn't really worth the bother as actual dates can stretch into the space underneath it. If you still want it gone here's how.

Firstly in Thunderbird go to Settings and scroll right to the end and click Config Editor. When that opens type:—

toolkit.legacyUserProfileCustomizations.stylesheets

into the search box that appears — you probably won't have to finish typing before it shows in the list below. If its value is false, double-click the word false and it will change to true.

Now press Alt-H followed by Alt-I. Look down the blue block to the left of the screen that will open until you come to Profile Directory. Click the Open Directory button next to it. This will open your Profile Directory in Windows Explorer, or whatever file browser your system uses.

Make a directory/folder in that called chrome, all lower case. Now get into that and create a text file called userChrome.css, again all lower-case apart from the C in the middle.

Into that file put this:—

#dateColSplitter, .button-column-picker { display: none !important; }

You need to re-start Thunderbird for the change to actually take effect.

Thank you for the very fast response and the detailed instructions - very helpful!

My right-most column is size and unfortunately it won't stretch into the space underneath the column picker. I followed your instructions - #dateColSplitter I changed later to #sizeColSplitter - and the column picker button as well as the splitter to the left of it is gone.

But Thunderbird does not use the freed up space. Instead it keeps an empty column about the width of the column picker button.

Do you have a remedy for that too?

Thanks again, Reinhard