I suggest that when choosing the text editor, the current value be displayed (and the selection box show the directory it's in), rather than being blank. The legend on the button used to access the selection dialog should then be changed from 'Change editor' to 'Select editor'.
Can you clarify what exactly you mean when you mention a selection box which is blank?
Greasemonkey Options --> click on button labelled 'Change Editor' (I'd like that changed to 'Select Editor') -->
This leads to a file selection dialog entitled "Please choose your preferred text editor first" displaying some seemingly random directory (perhaps %TMP%), and the file name field empty. If there already has been an editor selected, I'd like that selection box to have the name of that editor executable file and displaying the directory that it's in.
I agree on this. I pressed the Change Editor and all I see is my last downloads folder, and nothing preselected. It would be handy to somehow indicate the editor that is already set (In my case C:\Program Files\jEdit\jedit.exe), either as a text label in the settings dialog or after pressing the Change button.
Preselect existing setting when changing editor.