Is there a development tool plugin for Safari much like how there exists the Firebug plugin for Firefox?
Firebug is great, but Safari provides its own built-in development tools.
If you haven't already tried Safari's development kit, go to Safari-->Preferences-->Advanced, and check the box next to "Show Develop menu in menu bar".
Once you have the Develop menu enabled, you can use the Web Inspector to get a lot of the same functionality that Firebug provides.