When you paste a URL in the location bar or some text in the search bar, you most likely want to visit the URL or search for the terms you just pasted. So, why not paste and go to the web address, or paste and search the pasted terms?
In Firefox 4 you finally can. When you right-click on the location bar you will now get a Paste & Go option, and a Paste & Search option in the search bar. A simple way to save more than a few clicks!