The guys at Mozilla Labs released another cool extension for Firefox named Ubiquity. In their own words:"Ubiquity is an experimental Firefox extension that gives you a powerful new way to interact with the Web. You're used to telling Firefox where you want to go by typing Web addresses into the URL bar:
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With Ubiquity installed, you'll be able to tell Firefox what you want it to do by typing commands into a new Ubiquity input box."
If your are using firefox to see this post, you will notice that on the top there's a notification that this site has Ubiquity commands. This is my test on how easy it is know to create a extension to Firefox. This command allows you to subscribe (using Google Reader) to the feed on the visiting page using the 'subscribeThis' command.
So go on and give it a try!