Maybe everyone already knew about these, but I just stumbled on them a few weeks ago and found some more yesterday. You can have animated favicons in Firefox – tabs are animated and when you bookmark them, the bookmark icon is animated too. A quick peek at the HTML of a page with animated favicons shows they simply have a 16×16 animated gif and point the page to that as their favicon with this:
( NOTE: <> changed to  so this would show up)
[link href="images/favico.gif" TYPE="image/gif" REL="icon"]
I have also read that you can just rename your 16×16 animated gif as favicion.ico but that just seems wrong. I just tried that and it does indeed work though. I just took this animated gif and renamed it to favicon.ico and it works. Visit this page to see it in action. The nice thing about doing it this way is that you don’t have to go change all of your pages. It didn’t show up for me in IE though.
A quick and easy way to see how an image would look as a favicon is to simply view the image in Firefox. When you view an image in Firefox, it shows it as a favicon in the tab and in the address bar. This makes it easy to scour the web for a good animated gif as a favicon. Simply find an image and “view image” in Firefox to see how it would look.
Here is a good page to grab some animated gifs to play around with. Choose your style and put a letter in the edit field and hit submit then right click on the animated letter and save the image. That’s where I got both of the example images for my own tests.
Here are a bunch of example websites that have animated favicons. View them in Firefox to see them.