Just what I needed! Rated 4 out of 5 stars
I only found two minor issues with the add-on:
1) It doesn't seem to allow the evaluation of statements which are wrapped over more than one line. The script entry text area seems to wrap statements longer that 168 characters over multiple lines and when you select the whole statement they are interpreted as multiple statements which causes the evaluation to fail.
2)The "Properties" function under the Edit button seems only to work for some elements in the DOM, not all. For some elements null is returned instead of the properties list. Maybe this is a misunderstanding of the usage on my part, though.
Other than that it is a great little tool of use to JS developers.