From bug 1039881 comment 37 with bug 1039881 comment 24 providing some context for the crash. Brief summary is it seems that creating a background thumbnail for a new tab history tile could cause SetNewDocument to crash at mContext->WillInitializeContext(); It's currently not crashing on mozilla-central, so either it's fixed elsewhere or being masked by some other changes more recent than those for mozilla-beta. bholley suggests we can be proactively safer by landing the change we made on beta on nightly and aurora.

Dup of bug 1031303?

r=bholley from bug 1039881 comment 28

(In reply to Olli Pettay [:smaug] from comment #1) > Dup of bug 1031303? Quite possibly. I'll try to bisect around there to see if that caused the tests to no longer crash with the pref change from bug 1039881.

Thanks smaug! tests crash: https://tbpl.mozilla.org/?tree=Try&rev=ff7871d38d71 https://hg.mozilla.org/mozilla-central/rev/0174684f6250 Merge m-c to inbound. a=merge tests pass: https://tbpl.mozilla.org/?tree=Try&rev=105d98581b84 https://hg.mozilla.org/mozilla-central/rev/12b4cad8e0bc Bug 1031303 - mContext can be null when nsGlobalWindow::SetNewDocument is called, r=jst