Debugging is always hard, especially when debugging other people's code. Adding a bunch of console.log() statements, becomes tedious after a while and that's assuming that you are familiar enough with the code in question to know where to place them. Tools like Firebug are a tremendous help in this case. Having the ability to pause the execution at any point or inspect variables and stack frames, provides valuable insights into the runtime behavior of a program.
Firebug depends on the traditional JSD API in SpiderMonkey to perform its magic, but nowadays JSD is limiting in a number of ways. In order to overcome these limitations, the new JSDBG2 API was designed and implemented by Jim Blandy and Jason Orendorff, and the first version of this work landed in Firefox 8. Jim, being the kind of guy that has forgotten more about debuggers than I will ever know, designed a remote debugging protocol to separate the debugger from the browser, allowing for debugging desktop browsers, mobile browsers, mail readers and even more exotic software in the future. Dave Camp got a great protocol implementation up and running, but was assigned managerial duties (poor sap!) before having a chance to finish the job, so Victor Porof, Mihai Șucan and yours truly lined up to carry the torch. The culmination of this work landed in mozilla-central last week, including the remote protocol implementation and a prototype UI (disabled by default). It is now available in nightlies for adventurous souls to play with.
Let me be clear about that last point: what's in there right now is not useful for day-to-day work, even for experienced developers. There is no UI to add breakpoints besides the experimental Graphical Command Line Interface (whose awesomeness deserves a separate post), no stepping, no variable inspection besides call parameters and 'this', and numerous other limitations. In short: it's not ready, yet. In the course of the next weeks we will be furiously working to add the missing bits and improve the UI, so that it becomes a worthy addition to our existing suite of developer tools.
If, however, you are the kind of person who doesn't take no for an answer, here is what you need to do to play with it: type about:config in the awesomebar and set the preference devtools.debugger.enabled to true. Also, set devtools.gcli.enable to true in order to be able to set breakpoints. Restart your browser and select Tools -> Web Developer -> Script Debugger to open it. You will need to reload the page for the debugger to become aware of the scripts in the page (see? It's not ready!). Browse through the source scripts to find a place where you would like to add a breakpoint, then open the web console and type break add line <url> <line>. Don't worry about memorizing it, GCLI will guide you along the way, suggesting among the available options at each step. When the code hits the breakpoint, the debugger will display the stack frames and the variables in scope (not all of them, it's not ready, remember?) for inspection.
If you have better things to do with your time than play with experimental, uninished debuggers, but would still like to take a peek into what's in store already, here is a short screencast that should give you a broad idea. Note that the stepping functionality in the demo is not in the nightlies yet, because, you guessed it, it's not quite ready.
Ready or not, we would love to have any feedback you might have. Drop by #devtools for a chat, or file bugs to