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>
For example a To find the file that related to your project you need search the keyword at http://lxr.mozilla.org. First you need specify which version of Firefox your working on then do a search for the keyword, for example for our project "Canvas" which Canvas Rendering 2D /3D Objecton the screen.<br/><br/><br/>[[Image:Yshen6_searchCanvas.JPG]]<br/><br/><br/>There are hundreds of lines in different files related to Canvas, then we need read some commons or go in to code to specify which file we need, for our project is: nsCanvasRenderingContext2D.cpp, then we need go back to our source files and open it:
<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/>
