Snapmaze have created a Firefox extension that allows you to generate a QR code for the URL of the page being viewed. It’s a nice idea but slightly spoilt by the over-size window that opens to display the QR code and the Google ads shown below the code. You can download the extension here - Snapmaze Firefox Extension.

Just a quick comment to say that I created a Firefox extension for this purpose (and it now does a few other things with QRCodes). You can find it on my blog or by searching the Firefox addons site for barcode.
Snapmaze is a plugin for my browser that changes any weblink in a webpage into a QR-code for my N95. Pretty useful!