How to find bad CSS and javascript statements within Firefox?

It happens sometimes that I wrote and apply buggy CSS & Javascript/jQuery scripts into a webpage.

An errorneous CSS rule could be

.myclassname { width: 1024kk !important; }};

or a javascript

var pane = document.getElementsByClassName(“page-header-meta”)[0];

does not find at least 1 instance of the class.

So the remaining statements of such a scripts are not executed.

How can I find out (in Firefox) which statement is causing the trouble?

The webdeveloper pane at the bottom is not really useful.

I have problems finding in it errors or warnings which come from my focussed *.css or *.user.js script (errors from other *.css and *.user.js) should be excluded.

How can I achieve a better/faster error search?

Peter