can a Javascript debugger do "when this variable is read", break point there, like a C / C++ debugger?
I am calling some library function, and deep down, it is finally do something with that argument, and instead of manually finding where it is actually using that value, can i say, stop the code when the variable is accessed?
I'm not aware of any way to do what you're asking, but can you try passing a value which is likely to break the library code? Then you'll get an exception and that can be seen in any JS debugger.
Try defining a getter for your variable. A getter is a piece of code that runs whenever someone accesses an object's value. (This relatively new JavaScript feature is not available in IE, but if you're just using it for debugging in Firefox it should be fine.)
Your getter may simply return the value, but you can set a breakpoint in your getter. Or, even better, you can have the getter run the debugger statement to halt automatically.
Dan Fabulich does have a more solid solution with the accessor method.
But I recently noticed that newer versions of Firebug have the ability to break on property changes. Find it in the DOM tabs.