When in Customize mode this caused a blank window. I just had a green transparent background that matched the Windows Aero titlebar. On exiting Customize the window remained blank until I restarted Firefox. Had to disable this.
Does everything I want it to. (Some people should go back and update their old "doesn't work in [version]" ratings.)
You DO, however, need to fix your toolbarbutton re:the toolbarpalette (as it is now, it can screw up the sizing of a toolbar and even all of the other buttons on that toolbar--correctable via CSS, of course, but one shouldn't have to).
I really wish this was compatible with the latest version of Firefox 3.x and the Firefox 4 betas. (Also, on a side note... the screenshots actually make me wish I still had my old Windows ME computer...)
I tried to use Venkman to do what this add-on does, but I could never figure out how to make it work. This is straightforward and allows you to easily run script commands in any scope you need to. An indispensable tool for any extension developer.
One small suggestion/request: To simplify things, I think the Evaluate and Window buttons could be combined into one button that is a menu-button.
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.