The Twitterbar extension allows you to post directly to your Twitter account from the Firefox address bar. A small unobtrusive gray icon sits to the right of your address bar and all you have to do is write the text of your tweet and click on this button to publish it. Also, hovering your mouse over the button will tell you how many characters you have left.

You can also post by typing ' --post' right after the tweet text or hit the gray arrow when visiting a certain webpage to carry out a URL tweet (i.e it adds 'Currently Browsing: ' in front of the URL). Options for the extension include safe/secure mode, open Twitter in new tab after posting and the ability to change the URL tweet 'Currently browsing' text.