Firefox with Greasemonkey only: Unlike most other email clients, when you reply to a message in Gmail, the subject line doesn’t automatically become editable—you have to click the “Edit Subject” link in order to change it. If you edit your email subject in reply to messages frequently, but hate to have to reach for the mouse to start in Gmail, there’s now a Greasemonkey user script for you. Download the Show Editable Subject user script for Gmail and give it a spin.