If I make a content page set a favicon as a data uri to an svg, and have that svg include an image whose source is remote (ie wikipedia.org), then that data uri makes it to the parent process image element intact and we do get as far as creating a channel for that remote uri with private browsing not set. However, I think there is something that blocks the load remote image at some point after that because the image does not show up in the favicon when loaded in a non-debug build (to avoid hitting this assert).
Bug 1869938 Comment 9 Edit History
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If I make a content page set a favicon as a data uri to an svg, and have that svg include an image whose source is remote (ie wikipedia.org), then that data uri makes it to the parent process image element intact and we do get as far as creating a channel for that remote uri with private browsing not set. However, I think there is something that blocks the load of the remote image at some point after that because the image does not show up in the favicon when loaded in a non-debug build (to avoid hitting this assert).