Now we are ready to set a breakpoint in a source file so we can stop Firefox and inspect it running at the appropriate point. Open a source file

<pre>File > Open > File...</pre>

<pre>C:\proj\mozilla\content\canvas\src\nsCanvasRenderingContext2D.cpp</pre>

Then locate a line for your breakpoint and click to the left of the line number in the gray selection border then it set a red circle:<br/><br/><br/>