I really hate the tileset menu, the scroll bar is horrible.
It would be better if the list of tilesets appeared in a seperate window and have some kind of search function to find the set you want.
Also, how about having a preview image show up whenever a tileset is chosen?
And, at the bottom of the window have the usual "OK" and "Cancel" buttons.

BTW, I didn't know mice had horizontal scroll wheels.
Never seen those here in the UK.