This creates a file named cc-edges-NNNN.log and writes a dump of the heap known to the cycle collector, which includes JS objects and also native C++ objects that participate in cycle collection, to the file.
Since Firefox 18 (night builds after 2012-09-13) one can override the default location of the log files by setting the MOZ_CC_LOG_DIRECTORY environment variable. http://people.mozilla.com/~mleibovic/cc-dump.xpi is an addon for Android Firefox which uses that to save files to /sdcard. The code for this addon is available on [https://github.com/leibovic/skeleton-addon-fxandroid/tree/cc Github].
The file location changed in Firefox 12: