Firebug in Safari

defaults write com.apple.Safari WebKitDeveloperExtras -bool true

If you prefer, you can use the Property List Editor to set the WebKitDeveloperExtras property in the Safari property file, which is located here: ~/Library/Preferences/com.apple.Safai.plist (use Finder to locate the file and double click to open). You’ll need to restart Safari after making the changes to enable Web Inspector.