What is the version number of your Firefox and Kee?
Is the Matched entries dialog box entirely white?
Does it happen on all websites?
Does it make a difference whether you invoke the dialog box by clicking on the Kee icon in a form field, the main icon in the browser toolbar or the keyboard shortcut Ctrl+Shift+2?
You could try enabling debug logging in Kee options and then look in your browser console for any useful information. Unfortunately the new version of Firefox separates different parts of the log into different console views so it’s a little difficult to get a coherent debug log for sharing at the moment.