Fix new tab for ESR 140

See #43787 (closed) for the upstream changes. I think this probably broke our "new tab" button. It basically partially opens the new tab, but fails to load the page and probably has some errors. So the tab opens in a state where only the letterboxing background is visible (the xul:browser element is transparent).

This issue is just focused on fixing this new tab issue, rather than necessarily reviewing all of the upstream changes.