Not working with TB 6.0.2 Rated 2 out of 5 stars
Not working with TB 6.0.2 :-( :-( :-( ... then I found DOM Inspector 2.0.10 :-) that works... the TB Addon Developer docs point here, so I expect that need fixing.
Firefox? Rated 3 out of 5 stars
If you're looking for similar functionality with FireFox, you should of course use FireBug: https://addons.mozilla.org/en-US/firefox/addon/1843
Rated 5 out of 5 stars
You can get the one for Firefox at https://addons.mozilla.org/en-US/firefox/addon/6622
(Giving a rating of 5 to balance out the comments about it not working in FF since it is for Thunderbird, but it is confusing)
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.
Breaks Tabbing Rated 1 out of 5 stars
Dom Inspector made it so I could no longer tab through forms on my Mac. I\'m not sure if the OS matters, but I\'m using Leopard. This was enough of a problem that I never used it prior to uninstalling.