There’s a plugin – Firebug – to do just that on Firefox. Google Chrome has it built it, just right-click anywhere in Chrome and the menu is right there. Safari has it built in too! But it’s not activated by default.
Unless you carefully look at every option in Preferences for Safari, you would most probably miss it. Navigate to Preferences > Advanced and check Show Develop menu in menu bar.
Now, like Google Chrome, when you right click on any part of the webpage, you will see this.