I tried a build from 2010-12-25 (based on https://bugzilla.mozilla.org/show_bug.cgi?id=620773#c35) and it worked.
Looks like it is certainly caused by bug 611910 (to make things more confusing, the bugs in https://bugzilla.mozilla.org/show_bug.cgi?id=620773#c35 are reversed...bug 620773 landed in 2011, see https://bugzilla.mozilla.org/show_bug.cgi?id=620773#c23)
I tried a build from 2010-12-25 (based on https:/ /bugzilla. mozilla. org/show_ bug.cgi? id=620773# c35) and it worked.
Looks like it is certainly caused by bug 611910 (to make things more confusing, the bugs in https:/ /bugzilla. mozilla. org/show_ bug.cgi? id=620773# c35 are reversed...bug 620773 landed in 2011, see https:/ /bugzilla. mozilla. org/show_ bug.cgi? id=620773# c23)