Many tools use tinyurl to shorten URLs, because it was the origin of this wonderful idea. TwitterFox is such an example.
I prefer TwitterFox to twhirl because as a Firefox add-on, the url can be inserted into the tweet with a single click (Besides, word “twhirl” is hard to remember :) ). But why didn’t it provide an option of URL shortener to use?
We can modify its source to achieve this, though. Open the file chrome/TwitterFox.jar!/content/twitternotifier.js and find the line where TINYURL_CREATE_API is defined. Change the constant value to “http://is.gd/api.php?longurl=”, which is the API of is.gd.
There are countless URL shorteners around the web. Just choose your favorite.