This is spun out of this discussion.

UrlbarProviderOpenTabs sets a userContextId property on its results: https://searchfox.org/mozilla-central/rev/8526066f548af9ec3ebb462ff73c47ccc183f533/browser/components/urlbar/UrlbarProviderOpenTabs.jsm#181

The main doc also mentions it: https://searchfox.org/mozilla-central/rev/8526066f548af9ec3ebb462ff73c47ccc183f533/browser/components/urlbar/docs/overview.rst#383

But it doesn't seem to be used anywhere. Also note that QueryContext has the same property.

We may need this property to fix a few container-related bugs: bug 1479858, bug 1500991, bug 1538069. But it might be hard to fix. So for right now, if properly fixing this property is hard to do, we may want to just remove it for now and then work on it later when we start to address those other bugs. If we keep it, it should either be the effective userContextId of the tab, or at a minimum default to the one from the query context.

For now I'll mark this as blocking those container-related bugs.