Bug 1708530 points out that default icons in the Urlbar are different colours. While that bug fixed most of the icons, the icons on synced tab results are still opaque black. That's because the src on synced tab icons is moz-anno:favicon:page-icon<url>. We don't currently allow setting fill-opacity on icons loaded with moz-anno.

We expose -moz-context-properties to a limited set of protocols. We should consider adding moz-anno to the list. We should first confirm that we don't/don't plan to expose that protocol to content. We should add a MOZ_DIAGNOSTIC_ASSERT or something similar to confirm it is only served to chrome.