I have been using Safari for web development because of its great developer features. It has a DOM inspector, a resource tracker for tracking slow requests, even AJAX requests loaded after the page completes, a script debugger and a profiler and local database inspector. And best of all, no bugs that I have found yet.
I have heard that IE8s developer tools are also very good compared with Firebug.
It will also be interesting to see how Chrome shapes up as far as developer tools go.
Unless you just don't want to leave Firefox, or are using a strictly linux system, you might get better results doing your development in a different browser. At least that is what has worked for me.
Chrome uses Safari/WebKit's developer tools which are unhorrible, but has a couple of issues. (If you can simply paste to insert a new css attribute or DOM element, it would be amazing. Right now there's only hacky ways to do that.) What's interesting about it is that in the land of Google's minimal UI, is something that clearly belongs to Apple:
Are you using a Mac, by chance? The Mac version of Firefox isn't the best.
Linux is not Firefox only by any means - Google Chrome for Linux is very good, and the development tools are identical to the Windows and Mac versions of Chrome (which are the same Safari's tools). Opera for Linux is also top-notch.
Have you tried IE 8's dev tools? Firebug beats the pants off of IE 8's tool in usability and features, for me. Like most of IE, the dev tools are adequate, but lack a decent interface and are not updated or improved often, if ever.
I like Chrome and Safari's tools, but my personal assessment is that Firebug is more comfortable. It has all of the featured you mentioned except possibly the local database inspector.
For the site I work on the most, twice as much of our audience uses Firefox as Safari and Chrome combined, and FF even exceeds IE usage. So not only does it have my preferred dev tools, but is the most important browser for our audience.
I have heard that IE8s developer tools are also very good compared with Firebug.
It will also be interesting to see how Chrome shapes up as far as developer tools go.
Unless you just don't want to leave Firefox, or are using a strictly linux system, you might get better results doing your development in a different browser. At least that is what has worked for me.