I remember how excited I was to discover Firebug a few years ago, resulting in my almost abandoning IE as my initial development platform (you can't get away from developing for IE altogether because of its availability on every Windows computer).
My discovery of the Safari Developer tools was equally as exciting. For many of the most common problems that web developers face, Firebug was much more convenient and efficient than the development tools for IE.
The tools available in Safari are so sufficiently powerful and convenient to use that since I switched to Safari as my primary browser and discovered the integrated Developer tools, I haven't felt a need to fire up Firefox just to use FireBug.
If you prefer to use Safari for whatever reason, this little gem, the Safari Developer menu, will give you all the tools you need to be a top-notch web developer all in an integrated and well implemented package.
Happy bug hunting!