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.