From MozillaZine Knowledge Base
The article "Introduction to the DOM Inspector" provides a basic overview of the DOMi. It's a good place to orient yourself with the DOMi and learn how to use its features. The article has some Mozilla Suite-specific references, but most of the information still applies to DOMi in Firefox and other programs.
Getting the DOMi
Firefox 2 and earlier: The DOMi is included in builds since November 2003, as an option in the Windows installer. You must choose the "Custom" installation and select "DOM Inspector" in the "Choose Optional Components" window (see Installing Firefox on Windows for details). In at least some versions of Linux—Ubuntu for one—the DOMi is a separate package.
Starting in Firefox 3: The DOM Inspector is no longer included with Firefox but it is available as an extension.
DOMi was an installer option in Thunderbird builds since the end of March 2004 and in the 0.6 release, but it was removed from installer for the 0.8 release to keep download size small. It is now available as an extension—make sure you choose the right extension for your platform. (See bug 254031 for progress on an “official” extension.)
The bottom of the web page has a link to older releases of the extension.
Mozilla Suite and SeaMonkey
The DOM Inspector is available in the "Complete" Mozilla Suite and SeaMonkey installation or by selecting the Inspector component in a "Custom" installation.
Additionally, the Inspector Widget extension adds an "Inspect" toolbar button and the menu option "Tools -> Web Development -> DOM Inspector". Selecting an element after clicking the Inspect button will launch the DOMi and inspect the element.
For more related extensions, search for DOM Inspector on Mozilla Add-ons site.
Using the DOMi
This section is a stub.
Issues with the DOMi
Warning! Changing themes while using the DOMi may cause the program to crash.
NOTE There is a problem with the Inspector if "Show Whitespace Nodes" is not selected - > the "Blink Selected Element" option is turned off when the Inspector is opened. It does not store the preferences... There is a bug (307255) and a proposed "fix" at this location:
It seems to address the old Inspector that came as an extension, so no work around for now (?).