Support for the CSS Transitions module, originally proposed and implemented by the WebKit team, has just landed in the nightly builds of Firefox; it’s unsure whether this will make it into 3.6, as this is due to be released in the near future, but should be a feature in 3.7 at least.
The experimental feature will use the proprietary ‑moz- prefix; I’ve updated the demos in my previous post, Anime with CSS and WebKit, to reflect this.
After performing some brief tests there seem to be a few problems with the implementation; it’s not as smooth as it could be, and the third demo doesn’t work as it should. Update: Found an issue with the browser not recognising a value without a unit (e.g. 0 instead of 0%); fixed the demo, and filed a bug.