This is a spin-off from bug 640923 comment 5; that bug has become a mess. Steps to reproduce from SineSwiper: - load about:memory tab only, Safe Mode, full debug, and see the same result Where the "same result" is increasing memory usage. See bug 640923 for full details, some graphs, etc. It's possible that bug 650649 may be involved.
I also wonder if bug 656120 being fixed helps.
Forward dup'ing this to bug 692478 since that sounds the same and has more specific steps to reproduce.