The nightly builds on March 14 and onwards have a bug in the error console with regards to the garbage collection messages. To reproduce: 1) Ensure that the preference javascript.options.mem.log is set to true 2) Open the Error Console 3) Wait for a GC console message Expected (of the following variant): > GC(T+0.0) TotalTime: 254.2ms, Type: global, MMU(20ms): 0%, MMU(50ms): 0%, > Reason: MAYBEGC, +chunks: 0, -chunks: 0 mark: 160.2, > mark-roots: 5.8, mark-other: 3.6, sweep: 92.0, sweep-obj: 7.9, > sweep-string: 12.2, sweep-script: 1.2, sweep-shape: 6.7, > discard-code: 6.8, discard-analysis: 46.4, xpconnect: 3.5, > deallocate: 0.4 Actual: > GC(T+#) T

Henrik believes that it has something to do with this commit: http://hg.mozilla.org/mozilla-central/rev/4fa4374cf9de#l1.95

It's a regression from the API changes on bug 531396.