Rated 4 out of 5 stars

Basically required for anybody working with the firefox UI. Interface seems a bit half-baked sometimes, but it works well enough.

Should have a keyboard shortcut though, because some elements of the UI (alert popup for saving passwords, australis menu etc) disappear when they lose focus so it's impossible to select them.
A kind of workaround is to navigate via arrow keys and tab, but that goes against the simple point-and-click functionality..

Also, minor nitpick, but the icon doesn't show up well on dark backgrounds.

This review is for a previous version of the add-on (4.04.20121231.1-signed.1-signed). 

I've provided the following alternate solution to the need to open DOMi on popups, in version 4.38.20150523. Simply open the popup then hold the Shift+Ctrl keys and then click the desired popup element. I've updated the screenshots to depict the steps.