Hi Urs,
I've just finished a rewritten version. I tried to clean up my original code by making it less reliant on using closures - as well as being more careful about tidying up event listeners.
Aside from the change in coding style the other major change is the addition of the functionality from your version of the script

I added your groups of CSS properties as a drop down list, and also included the ability to set a CSS property.
Some other little enhancements...
it automatically converts the rgb(0, 0, 0) colour values reported by mozilla into #000000 style values (I find these easier to read

)
I think the drag and drop behaviour works a little better
typing the word "empty" into the textbox and pressing enter clears the list of styles currently displayed. shift clicking on an item in this list removes that item
the view/edit CSS panel now also includes the id and class information of the element in its title
it has some tooltips in some places as well as some slightly better CSS.
and maybe a few other things I've forgotten
It works in Opera and Firefox/greasemonkey. Please try it out, suggestions are welcome!