Review for DOM Inspector by Brett Zamir
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)
To create your own collections, you must have a Mozilla Add-ons account.