The Problem With Dynamic JavaScript
However, what if the JavaScript file where you need to set breakpoints is not static but instead dynamic (generated on the fly). If you set a breakpoint in this case and reload the page, the breakpoint will most likely disappear, especially if the JavaScript url is generated uniquely every time.
The Solution
If you have access to the source, the solution comes in the form of the
debugger;
keyword. Just add it to your dynamic JavaScript generator or into any JavaScript file you have access to exactly where you want Firebug to break, and voila – it does.