Xenotix KeylogX is a keylogger add-on for Mozilla Firefox. It captures and logs keystrokes sent to the browser and you simply type alt-X to retrieve the data from the logfile.
You can download it from the following link: https://packetstormsecurity.com/files/download/110144/xenotix_keylogX.xpi
Source: https://packetstormsecurity.com/files/110144/Xenotix-KeylogX-Keylogger-For-Firefox.html