(You could also potentially fix this via bug 1381707) When creating a windowless parent process browser (to host a doc that has a remote XUL browser to host the child process content), we: 1) create the windowless browser 2) somehow trigger the initial about:blank load (I haven't investigated this in depth) 3) force creating another one with the system principal ( https://searchfox.org/mozilla-central/rev/0640ea80fbc8d48f8b197cd363e2535c95a15eb3/toolkit/components/extensions/ExtensionParent.jsm#1113 ). 4) immediately load a chrome: URI with a system principal triggering principal. I think esp. now that we explicitly pass system principal for the latter, I'd expect us to be able to just remove the aboutblank creation. We may also be able to remove (2), but I'm not sure without investigating further.
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