IndexedDB in Gecko has a couple different but related logging capabilities. We can hook into the Gecko Profiler to show data related to the performance of the database threads. These are enabled unconditionally in builds that support profiling.  You can find the points marked with a PROFILER_LABEL annotation in the IndexedDB module.

We also have a logging system that is off by default.  By setting the preference dom.indexedDB.logging.enabled and NSPR_LOG_MODULES=IndexedDB:5 you can enable the IDB_LOG_MARK annotations that are scattered throughout the IndexedDB code.  This will output a bunch of data (using the standard logging subsystem) to show DOM calls, transactions, request, and the other mechanics of IndexedDB.  This is intended for functional debugging (instead of looking at performance problems, which the PROFILER_LABEL annotations are for).  You can even set dom.indexedDB.logging.detailed to get extra verbose information.

http://blog.kylehuey.com/post/142708997842