Now all of the browsers have (or will soon have) their own debuggers. Basing next gen work on Firebug is not practical. The pace of change in browsers is too fast for our team size. Firebug hasn’t really been able to keep up with Firefox, let alone compete with other tools. Moreover, the shift from desktop to mobile and from one high-tech browser (Firefox) to three or more requires additional development effort. I think our current drive to re-architect Firebug is on the right track, but I could not obtain another year of support from IBM to contribute to that work.
Things don't look good for Firefox, with a lot of key visionaries leaving Mozilla over the last 6 months ... Yet, from the other point - WebKit might [already] be "a new Gecko", with fanboys, google, "hackers" and 3rd party devs (like Adobe) pumping out tonnes of code for it.