Since Safari 3.x, you can now have a firebug-like tool in Safari. Either open terminal and copy-paste the line below or edit the plist file here ~/Library/Preferences/com.apple.Safai.plist.
defaults write com.apple.Safari WebKitDeveloperExtras -bool true
Most websites stop there saying “you should now be able to right click and see the ‘Inspect Element’ in any contextual menu”. Well, I didn’t. I had to show the developer menu. Here’s how to show the developer menu straight from Safari help:
open Safari preferences, click Advanced, and select “Show Develop menu in menu bar.”
Once you do that you should be able to see “Inspect Element” in the right click menu.