Happy days again - the Javascript framework patch [#388564] for Javascript tracing in mozilla has landed in CVS head 
It's already being actively used:
"No problem -- dtrace has been a huge help for us, from what I can see. Sayrehas been helping front-end hackers identify JS performance problems with it,which would be hard to find otherwise." Brendan Eich
"I don't think I can work on Mozilla without DTrace ever again. Too useful." Land of DTrace, Robert Sayre
Mike Shaver nominated the dtrace team at Sun "for giving us the first good tool
we've ever had for understanding front-end performance." - Thanks Mike
Here are a few of the bugs it's already helped with, thanks to Robert Sayre's enthusiastic evangelism , cheers Robert
[#398300] instantiates livemark service on startup
[#398360] speed up onSecurityChange
[#398950] contentprefsink event registration
[#398344] utilityOverlay.js isElementVisible() is slow
I've built this from head on nevada build 74, having applied the gtk patch disconnect_handlers_and_prevent_unloading_v1.1 Unfortunately Firefox is now crashing in pango font hanlding, unrelated to the probe work 
Apparently this is ok in build 75 onwards, thanks again to Ginn Chen for the info, so will upgrade today and see how we get on. Also noticed that Johnny had to change some of the build framework to land the JavaScript patch and this has now broken the load probes patch. Ah well, bit more work for me ...
I've included some info on the JavaScript probes below. There are plenty of DTrace examples using the probes in the new 0.99 DTraceToolkit Brendan has just posted. Just unzip and look under the JavaScript subdirectory, I've referred to some of them below.
javascript provider probes
javascript*:::function-entry
Args: (char *filename, char *classname, char *funcname)
javascript*:::function-return
Args: (char *filename, char *classname, char *funcname)
The probe function-entry fires when a JavaScript function begins to execute. The probe, function-return, fires when the function completes. The probe arguments are still being designed, for now arg0 is a the file pathname, arg1 is a classname (if available), and arg2 is the function name. [See js_calls.d, js_calltime.d, js_calldist.d, js_flow.d, js_flowtime.d, js_stat.d, js_who.d]
javascript*:::function-info
Args: (char *filename, char *classname, char *funcname,
int lineno, char *runfilename, int runlineno)
Used to get line number information. The function-info probe is best for anonymous function tracing - since it gives line numbers for identifying the anonymous function location. [See js_flowinfo.d]
javascript*:::function-args
Args: (char *filename, char *classname, char *funcname,
int argc, void *argv, void *argv0,
void *argv1, void *argv2, void *argv3, void *argv4)
javascript*:::function-rval
Args: (char *filename, char *classname, char *funcname,
int lineno, void *rval, void *rval0)
So function-args was to provide some debugging visibility for function arguments; and function-rval was for the return value. [No examples in the toolkit as these are still experimental].
Status: args & rval currently return zero args in error, when in-built methods hit a new performance optimization in the JS engine. It needs fixing. If you try using them now, you'll see that they mostly work (especially for your own functions).
javascript*:::object-create
Args: (char *filename, char *classname, uintptr_t
*object, int rlineno)
Fires when a JavaScript object has been fully created. [See js_objgc.d]
javascript*:::object-create-start
Args: (char *filename, char *classname)
javascript*:::object-create-done
Args: (char *filename, char *classname)
Matching probes can be used to time object creation. [See js_calltime.d, js_cpudist.d, js_objcpu.d]
javascript*:::object-finalize
Args: (NULL, char *classname, uintptr_t *object)
Fires when an object is garbage collected. [See js_objgc.d]
javascript*:::execute-start
Args: (char *filename, int lineno)
javascript*:::execute-done
Args: (char *filename, int lineno)
These trace when a JavaScript program or block of code begins execution and ends. [See js_execs.d]