QR Code Image Generator lets you Create a QR Code of the selected text or current URL. You can customize images with different colors and sizes. New to this add-on is a shortcut with CTRL-SHIFT-Q to quickly see the QR Code for the current URL. Also possible to create a QR Code from free text within Firefox.
Want to learn more about this add-on? You can pick up the QR Code Image Generator from the Firefox Add-ons website.