Firefox users looking to shorten their Twitter messages to fit inside the service's 140-character limit should check out TwitterEyes. This new, experimental extension takes a look at what you've written and shortens any words it can. This includes any extra spaces you might have put in, along with words like "and" which gets turned into "&" as well as "first" which becomes "1st." In future versions developer Danny Pier says he intends to add a user-controlled dictionary that would allow you to add your own words and their shorter replacements.
In its current iteration, the extension adds a little TwitterEyes option next to the send button that only shows up when you're visiting from Twitter.com. I found it to work pretty well in a new Firefox window, but after using it once it didn't work without closing the tab and re-opening a new instance of Twitter.
This is very reminiscent of all the URL shorteners and shortening extensions that can be used to make links smaller. Considering how many users send tweets from mobile and desktop applications, it would be nice to get this same functionality available from there as well. This is a great first step though.