QR codes can be a very convenient way to encode links, images and other information, so they're easily accessible to just about anyone with a compatible smartphone (or similar mobile device).
You don't just have to use other people's QR codes, though. You can also create them yourself, and there are few easier ways to do this than with the Firefox extension QrCodeR.
After a straightforward installation, you'll find the program has added a new "Encode... to QrCode" option to the right-click menu for text selections, images and URLs. Or, if you right-click an image which also has an associated link, say, then you'll see both an "Encode URL to QrCode" and an "Encode Image to QrCode" option. And there's even an icon in the address bar which will create a code for the current page URL.
Choose one of these options, and QrCodeR will display the appropriate QrCode in the centre of your screen. Hovering a mouse cursor over the code reveals its original URL. And clicking it allows you to save the code as a PNG file, ready for use wherever you like.