Whether you know it or not, I am a Firefox Extension Developer and have about 7 or so extensions that I've written for Firefox over the last couple of years. Some are functional, such as the Google Calendar Watcher extension, or the Codesnipp.it extension (for users of http://codesnipp.it). And others were created as a little bit of fun, such as the Profanity extension, which puts random swear words on any web page throughout any text!
My most popular extension by far is the Google Calendar Watcher, and it's definitely the one I've been most proud of as I use it on a daily basis. However, it has taken me a long to finally perfect it enough to a). be happy enough with it to release it, and b). for the very strict extension testers to approve it! But, I'm very happy to say that the new version is now available for download on the Firefox Add-on website and can be found Google Calendar Watcher.