This post hits a nerve. I've relied on Firebug for a long time and it's been getting less stable for about a year. The problems are particularly apparent if you have largeish Javascript files (a few thousand lines). A lot of critical features intermittently stop working, things like breakpoints or being able to jump to the definition of a function. The Firebug project shows signs of having hit that disastrous inflection point where bug-fixing efforts create as many bugs as they fix.

I suspect the root of the problem is that Firebug started as a nice little ad hoc add-in, attempted to evolve into a production development tool, and is breaking down under the strain. That plus the personnel changes on the project would explain what we're seeing.

It's bad because none of the alternatives I've tried are an adequate replacement yet.