I looked at https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/tabs/Tab
I use
browser.tabs.get(tabId)
- but the returned value is status:undefined and index: NaN
however the id is correct, as
browser.tabs.remove(tabId)
closes the tab in question.
What I intend to find out is, weather the tab with tabId has rendered content. This is slightly different from determining the status or URL i think, because when a tab is redirected there is an URL but no rendered content)