JS profilers are very useful tools for web developers, and other browsers (IE, Chrome) already have them. It would be great if bhackett's profiler could be added as a devtool, so that people don't just do profiling on other browsers - people optimize their code for browsers that have profilers.
I'm not sure a PC-counting based tool can count as a profiler to most web developers (which would expect a profiler to provide precise timing and function-level statistics IMHO).
There is lots of work going on for our upcoming debugger - we'll certainly want to integrate tools based on PC-count information in it at some stage (eg. for coverage information, like in the initial prototype).
I have been asked to get together a bunch of information so that I can create a feature page for a general profiler. The tool will profile at least JS & CSS, but as Cedric said, code coverage belongs in the debugger itself.
IMO we should get this tool out ASAP. If it wants to be integrated into other tools down the road, that's fine, but we've got cool and useful technology here, and we can get it in developers' hands fast. Let's not sit on it.
Cedric is looking at this.
For interested people reading this bug, this is available on AMO as JIT Inspector: https://addons.mozilla.org/en-US/firefox/addon/jit-inspector/