I usually hide the address bar in Firefox to save real estate, preferring instead to hit ctrl + L to pull up a popup address box. Often, though, I neede to copy the URL of the page I'm on, and there's no easy way to do it without the ability to highlight the address bar. I went looking for a Firefox addon and found CopyURL, a simple addon that adds a context menu item that, when clicked, copies the current URL to your clipboard.
It's a very useful addon for a very limited purpose. The addon's big brother, though, provides the same functionality and does one better. Copy ShortURL also adds a context menu item that allows you to copy the current page's URL to your clipboard, but it also shortens the URL so you can easily pop it into a tweet, a forum, an email, or any number of similar use cases.
The use of "canonical" shorteners makes the addon that much more elegant. If the site you're on uses built-in URL shortening, the addon will use that shortener. If it doesn't, the addon will default to the is.gd shortener. So if, for example, you're on the New York Times or New Yorker's website, or Techcrunch—all of which use their own URL shortener—those will be used. If not, is.gd will be used.
I'm guessing that not everyone will find this addon extremely useful, but for those of us who do use it, it'll likely be used pretty heavily.