Here's a profile I collected using talos + tscrollx locally: http://people.mozilla.org/~bgirard/cleopatra/#report=d31a5c7172f44819c8bea57452641bbe375a06f0 There are lots of gaps in both threads, some up to 15ms long. (The giant gaps are intentional and caused by pausing+resuming the profiler from the test.) 15ms of missing samples make a profile pretty useless. I collected another profile under the same conditions, but with bug 1150252 backed out: http://people.mozilla.org/~bgirard/cleopatra/#report=0a394a062b9fdf190ae827f02f97032d39d078e9 This still has some gaps, but those are usually just one missed sample, and not 15. From that we can conclude that bug 1150252 is the cause.

A build with bug 1150252 backed out is at http://ftp.mozilla.org/pub/mozilla.org/firefox/try-builds/mstange@themasta.com-82846c58774b/try-macosx64/firefox-41.0a1.en-US.mac.dmg .