Greasemonkey Tutorial
An alternative to the iMacros approach is by using the addon Greasemonkey to accomplish the same thing. This approach may be a bit easier to install for less technical users, but I still prefer iMacros because it will prompt you for the captcha text. Nevertheless, here is a tutorial for the Greasemonkey approach.
First, go to http://addons.mozilla.org
Then, search for 'Greasemonkey'
Click 'Add to Firefox'
Next, go to:
http://userscripts.org/scripts/show/46820
Click 'Install'
Go to http://twitter.com, and the rest should be done for you. All you will have to do is enter the captcha image text. When you are ready to uninstall, click here
Except, instead of uninstalling iMacros, uninstall Greasemonkey... but you knew that, right?