Rated 1 out of 5 stars
It looks like this plug-in is for Thunderbird, not Firefox. This is especially stupid because a google search for "firefox dom inspector" reveals this as one of the first results and when you click on it, the heading on the page says "Firefox Add Ons".
For Firefox, it looks like you need to run the installer again - http://kb.mozillazine.org/DOM_Inspector
Please make the DOM Inspector available as a Firefox add-on in addition to an installer plugin -- it's stupid that I need to reinstall firefox to get this to work.