I guess this won't be a huge miss for most people, but just a heads up that Mozilla's upcoming Firefox 18 browser will no longer support animated themes due to new performance updates packages in that version. Neowin says Mozilla is looking into other solutions that might bring back animated themes at some point, but users who absolutely need their animated theme fix can download the Personas Shuffler add-on to preserve the effect.
Themes, formerly called "Personas", require large image files to accommodate the wide variety of screen resolutions and header heights. These images - 300 x 200 px for headers and 3000 x 100 px for footers - slow down the loading process for Firefox. Of course, animated themes had an even greater negative effect on load times and performance, since the browser has to redraw the huge images continuously.
The solution in Firefox 18 is to crop the images to the user's screen size at start-up, which improves load times by a significant amount. Bug 650968 has the full details of this change. A side effect of this solution, though, is that animated themes will no longer animate in the new version of Firefox.