Safari's Web Inspector is far, far better than it used to be, and has indeed gone a long way towards catching Firebug. The problem is that it isn't there yet; otherwise, I'd only be dipping into Firefox to test things.

The Web Inspector's major shortcoming is the lack of a UI to edit the some aspects of the DOM directly (you can do it in the JS console by running DOM queries). One can't remove existing nodes or add new ones via the Web Inspector UI; nor can one add new CSS styles (although one can edit node attributes and the values of existing CSS declarations).

All of this means that even for those of us with access to Safari (i.e. not developers working on Linux) there's still a pressing need for Firebug, so if they keep breaking it with updates pretty soon everyone will be reverting to 3.0.x and the older versions which, while they had a few niggles, essentially worked.