Using Mootools 1.4.4 breaks the animation and just resizes to 200px. Mootools 1.4.3 shows the animation.
Tested with FF 10.0.2 and IE 9.
Thanks @matthiasstock, but this is a known issue and recently resolved in the master branch. Take a look at: #2282.
Please give us a hand and test the master branch with your code.
Here are the instructions on how to build mootools: https://github.com/mootools/mootools-core/wiki/Build-MooTools-from-git-master or use this nightly build: http://mootools.net/nightly/build/mootools-core.js