This affects tpaint. Effectively, what happens is: - nsWindowWatcher::OpenWindowInternal creates the window - (cached, because the new window is same-process with its opener) frame scripts run, touching content (bug 1471327 and friends), forcing an about:blank window to be created - nsWindowWatcher::OpenWindowInternal continues running, and this code: https://searchfox.org/mozilla-central/rev/0640ea80fbc8d48f8b197cd363e2535c95a15eb3/toolkit/components/windowwatcher/nsWindowWatcher.cpp#1086 sets the principal of the about:blank window, and to do that, re-creates the about:blank content viewer. The comment says that "this is safe to call multiple times". That might well be true, but doing the work twice is wasteful... Longer-term, we could also look at whether we can avoid creating about:blank if the originating document doesn't pass a (non-special, ie not "_blank") window name and passes a non-principal-inheriting URI and doesn't use the return value - but that's a harder problem.