Mozilla announced Tuesday on its Add-Ons Blog that animated themes would no longer work in the upcoming Firefox 18, due to performance updates packaged in that version.
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. Truly a tragedy for user customization.
The Firefox team is looking into other solutions that might bring back animated themes at some point. However, users who refuse to go gentle into that good night can install the Personas Shuffler add-on, which will preserve the animation as it refreshes the themes on every load.
Source: Firefox Add-Ons Blog