Profiling with the gecko profiler
See these instructions. Patches are in-flight to get native stacks in profiles, but that's not in default configurations yet.
Profiling with systrace
Good at: Shows process preemption, shows all calls to instrumented functions, Familiar for android developers
Bad at: Requires configure option, higher overhead
Better instructions coming: Build with '--enable-systrace' and follow these instructions for systrace.py.
Note: If you're profiling on device with Android 4.3 and sometimes you can only see B2G process in your profile data, please reboot your device and try to profile again. By the way, gecko code is tagged as 'gfx', so the command is for your reference.
./systrace.py --time=10 -o mynewtrace.html sched gfx