Using developer tools on a Thuderbird add-on dialog

I was having trouble getting Thunderbird's developer tools inspector to focus on an add-on's dialog box, since the usual technique of using the "pick an element" tool doesn't function there, nor do you get the "inspect element" option on right-click in the iframe. So, I thought I would post the solution here to help others.

The trick is to use the "Select an iframe" button on the right side of the toolbar:

This will show a long list of iframes, one of which should be your add-on's dialog in the format:

chrome://myaddon/content/myaddonDialog.xul

Unfortunately, you still have to dig around through the tree to find the right elements, but at least they are highlighted in red on the iframe while you are searching.