TwitterBar by Christopher Finke allows you to post to Twitter from Firefox’s address bar. A small Twitter icon sits to the right of your address bar; clicking on it will post your tweet, and you can hover your mouse over it to see how many characters you have left.

You can also post by typing “–post” at the end of your tweet. Clicking the Twitter icon when visiting a webpage will send a tweet containing the URL of the webpage you are currently viewing.