Caterogy generation didn't change in Efficy 10, so the HTML select is still in use but the module Select2 is apply on.
So I will say you may keep the same code and make it ready for Efficy 10 by using Jquery instead of Mootools.
Carefull, the HTML id of the field in Efficy 10 in a category contains now a dash (-) instead of a space ( ). ie:
Efficy 2012 : id was "CATEGORY COMP$ADDRESS STREET"
Efficy 10th : id is now "CATEGORY-COMP$ADDRESS-STREET"