In order to accommodate various screen resolutions and header heights, themes (formerly Personas) require very large image files. Header images need to be 3000 x 200 px and footer images 3000 x 100 px. As a result, loading them slows down Firefox. Animated themes pose an even more persistent problem for performance, because animated images are much larger in size, and the browser needs to redraw them continuously.
Bug 650968 was filed to address this issue, and the solution was to crop the images to your screen size when you start Firefox. Because smaller images are loaded when the browser starts, load times are much improved. However, a side-effect of the cropping is that animated images are not taken into account. For this reason, animated themes will no longer animate starting in Firefox 18. Since this was a unintended consequence, there are some edge cases where the image will animate normally until Firefox is restarted.
We understand that animated themes are very popular with some users, and people will be upset with this difficult decision. However, to keep Firefox performing optimally for the even greater number of users who care about speed, the decision was necessary. We would love to find a way to preserve animated themes, and are currently looking into alternative solutions. In the meantime, a workaround would be to install the Personas Shuffler, which will refresh your animated themes every time Firefox loads, keeping them animating continuously.
There are still many beautiful and creative themes available to personalize your Firefox, with more added every day, and we hope you continue to enjoy them.