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.
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.