On a recent Mozilla-Central, when setting XPCOM_MEM_LEAK_LOG=leaklog.txt, loading about:blank and then shutting down the browser within say 5 seconds gives me this in the leak log: |<----------------Class--------------->|<-----Bytes------>|<----------------Objects---------------->|<--------------References-------------->| Per-Inst Leaked Total Rem Mean StdDev Total Rem Mean StdDev 0 TOTAL 36 416 220067 6 ( 1739.29 +/- 2779.26) 599137 2 ( 1800.12 +/- 3379.56) 82 Mutex 24 48 223 2 ( 61.34 +/- 21.75) 0 0 ( 0.00 +/- 0.00) 105 ReentrantMonitor 32 32 91 1 ( 26.89 +/- 13.50) 0 0 ( 0.00 +/- 0.00) 590 nsTArray_base 8 8 21478 1 ( 6763.06 +/- 3480.13) 0 0 ( 0.00 +/- 0.00) 597 nsThread 224 224 8 1 ( 3.73 +/- 1.67) 410 1 ( 50.26 +/- 21.41) 601 nsTimerImpl 104 104 47 1 ( 15.49 +/- 6.38) 257 1 ( 34.16 +/- 11.74) If I don't do anything and let it sit there for a minute or so, then when I shut down, eventhing but the Mutexes is cleared out. For me, this happens 100% of the time. Jesse said he was seeing things like this, but only sometimes.