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It looks like the sliderfilter.js file was never properly updated from MooTools 1.1 to 1.2. The script here, which is used in the filter edit view, needs to be updated to work properly with MooTools 1.3.
Review sliderfilter.js #29
Partly fix for #29
There's still an issue related to the scroller that I couldn't figure out so far but at least the basics are working...