Recently I improved my XUL and JavaScript skills and wrote a small XUL application. To browse some data from a database. While at it I tried to create a list with resizable columns. While trying to get some help one of the suggestions was not to use a tree instead of a listbox to show a list. But well, that didn't solve the issue, the next important thing is to add a splitter betwwen the column headers, but well that alone would be too easy: You also have to set the CSS class to tree-splitter... So that's what such a tree looks like:

Thanks to Arne and Uwe for answering my stupid questions :-)