Вот общее решение:
var elem = /*...*/
var transformations = getComponentsFromAttribute(elem.style['-moz-transform']);
transformations.skew = [1, 1];
elem.style['-moz-transform'] = getAttributeFromComponents(transformations));
Использование этих функций:
function getComponentsFromAttribute(attribute)
{
var componentMap = {};
var regex = /([a-z]*)\(([^)]*)\)/gi;
var matches;
while(matches = regex.exec(attribute))
componentMap[matches[1]] = matches[2].split(/, */);
return componentMap;
}
function getAttributeFromComponents(componentMap)
{
var componentList = [];
for(var key in componentMap)
componentList.push(key + '(' + componentMap[key].join(',') + ')');
return componentList.join(' ');
}