Firefox extension question

Is it possible to make a Firefox extension so everytime you access a certains website you also see a box with an information, either integrated in the website or not. For example everytime you access google homepage you get a 开发者_如何学运维box with some information independent to the searches you make. Thanks a lot

Yes, there are a couple of approaches you could take.

You can avoid the complexity of doing an extension if you simply write a Greasemonkey script. There are a ton of example scripts at userscripts.org that you should be able to search for ways to approach your particular project.

If you want to distribute it as an extension, I'd suggest using the Addon SDK. It makes it really easy to develop and package an extension, and provides a framework called page-mods to modify pages.