Related:FireBug- A Firefox extension to Inspect HTML CSS and modify style in real-time.
For Firefox you can download the plug-in from here.
You can set a break point by locating the line you are interested in and click once in the left margin of the Source Code view. This will put a breakpoints at that point.
See the screen-shot below.
If the breakpoint appear as a solid red stop sign indicates that the breakpoint has been set.For future breakpoints it will display as ‘F’.A Future Breakpoint means that Venkman was unable to set a hard breakpoint now, but if the file is loaded later, and it has executable code at the selected line number, Venkman will automaticaly set a hard breakpoint.
To learn more about Venkman visit here.
To know more about Venkman project read it here.