Firefox: URL still often about:blank

The title in that case is set to the URL was typed in.

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/onActivated
> Note that the tab's URL may not be set at the time this event fired, but you can listen to [tabs.onUpdated](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/onUpdated) events to be notified when a URL is set.