This extension adds an instant reply button, which you can set after clicking the extension's icon.
This extension makes it a bit easier to reply to tweets with a default reply. Go to twitter.com and and click the icon. Add the text you want to use as a default reply and click save. Open the tweet you want to answer to. You will see that a button is added to do an instant reply. Click the button and your default reply will be added to the reply section.