how to overwrite css height set by jquery/javascript?

Your variables don't match; remember that punctuation and proper spelling are important to calling the variable properly; try changing the second line to:

$("#someElement").css('height',setHeight+'px !important');

Your variable name doesn't have a leading $. Also, the !important flag will cause this not to work in Firefox, however as you're applying this style directly to the element, you shouldn't need it.

$("#someElement").css('height', setHeight + "px");

Also note that if you're only setting the element's height you can also just shorten this to a height() call:

$("#someElement").height(setHeight);

setHeight, not $setHeight. and the !important is unneeded.