Dragonfly is slow to start up compared to the other two. Also, opening Dragonfly for the first time in a session necessitates a full HTTP hit to reload. Firebug and Chrome don't suffer from that problem.

Firebug and Chrome let you right click on a page to "Inspect Element", which brings the DOM inspection tree right to the right-clicked element. Dragonfly had previously not had this, but they just added it, according to the OP: "One of the biggest usability issues has also been solved, with inspect element being available from the Web page context menu."

Firebug and Chrome let you toggle individual CSS rules. Dragonfly has no such facility [yet].

Firebug and Chrome let you search the DOM using CSS or XPath. In Dragonfly, you can only do a plain text search.

Overall, Firebug and Chrome have that "native UI" feel, but Dragonfly really feels like it's running Javascript.

I still think Dragonfly has a ways to go to catch up to the others, but it's good enough for me to pop it open if I just want to quickly inspect the HTML tree. For more complicated web debugging, I turn to Chrom[ium]. But now that Dragonfly is open source, I'm certainly going to explore the code, and possibly even contribute patches.