The openerTabId is present when the user clicked a link in the tab or the tabs js opened it. Neither is the case for webext popups. The user opens the popup using the browser, not the tab. I would say this seems correct. If you want to get the tab the user is currently on, just query the active tab when the popup gets loaded.
So it’s not a popup opened by the WebExtension. The WebExtension only (later!) intercepts the load request and redirects it, but that should only be done afterwards, and of course should not change the tab data.
Also the WebExtension does the same for the new tab and the new popup, but only in one case, there is no openerTabId…
Sorry about the delay. I guess I misunderstood… I thought you meant the extension popup. Maybe the openerTabId is only supplied if it’s within the same window. But it sounds like a bug to me. Have you created a bugzilla ticket for this?