Similar to the flipping IE prank, there is a way of doing it in Firefox. The theory is the same, but quite a different way to reach it. A bit harder, but still a great effect.
First – find the Firefox user profile. This can be tricky, since there are so many places to store it. This is the easiest way (I think…):

If you are using Windows, then start Windows Explorer and type
%appdata%
and press Enter. This will bring you to the folder where the user’s settings are stored. On other operating systems, the path will be different.

The next step is to find the Firefox profile. This is somewhat different depending on your version, but in this example we will use Firefox version 5.

Open the folder Mozilla/Firefox/Profiles. Here you will find one folder per user profile. You will probably only find one folder here – if there are more, you will have to dig a bit to find which one it is. Enter that folder.

Now, create a folder named chrome (has nothing to do with the competing product). Here we will create a file named userContent.css and put some styles in it.

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   Open the Firefox user profile folder (somewhere like C:\Users\UserName\AppData\Roaming\Mozilla\Firefox\Profiles\zweeredf.default)
2. Create a folder named chrome

Now we continue

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   body {
      -moz-transform: rotate(8deg);
   }

4. Save it as userContent.css in the chrome folder, and as always, make sure you have the right file extension<
5. That’s it. If you’ve done it properly, Firefox should look something like this after next restart

And here are some variations of the style sheet:

Everything fades out when the mouse pointer is over the browser

body {
  -moz-transition-property: opacity;
  -moz-transition-duration: 0.5s;
  opacity: 1;
}

body:hover {
  -moz-transition-property: opacity;
  -moz-transition-duration: 0.5s;
  opacity: 0;
}

Images rotates when hovering them

img {
  -moz-transition-property: -moz-transform;
  -moz-transition-duration: 0.5s;
  -moz-transform: rotate(0deg);
}

img:hover {
  -moz-transition-property: -moz-transform;
  -moz-transition-duration: 0.5s;
  -moz-transform: rotate(360deg);
}

Changes in the style sheet are applied when you restart the browser