Twitter Links is a small firefox extension which aims to enrich the user experience on the twitter website. Shortened urls are lengthened asynchronously and the extension runs on page changes, useful on twitter where much of the user experience is driven by Ajax calls to the server (no actual page refreshes).
The add on is available from the Mozilla addons site.
Now up to version 0.9.6 with support for photo previews from yfrog, twitpic, tweetphoto and twitrpix. The aim is to keep the addon as non-intrusive and lightweight as possible, with no real need for user preferences. Feature requests welcome.
Bugs – let me know: mozilla at dontstopnow dot co dot uk or leave a comment
UPDATE: This project ceased to be useful and I stopped updating it when Twitter themselves added all of the features.