Some of the features include:

• Search right from your Twitter home page
• Including all replies (including tweets that your @username is in the middle of the tweet instead of just at the beginning) on the replies tab
• Expanding tinyurl style links to include the title of the page they link to
• Embedding media directly in the Twitter interface instead of only through links (like twitpic)
• Providing a mouseover on a user’s avatar to show their recent tweets (giving needed context sometimes)
• And a few other nice odds and ends…

I included a few screenshots of some of these features above.

Overall this has proven to be a really great plugin from my usage so far. I find a lot of it is stuff that I didn’t realize how much I would use until I have it. I think it’s great that the Twitter community is active enough to modify the Twitter interface using FireFox addons, but it does raise a few questions about why the Twitter team isn’t integrating this stuff themselves.

In any case, if you use Twitter from the interface at Twitter.com much (and you browse in FireFox) I’d definitely recommend giving this addon a try.