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WebExt code can be run from three contexts:
Content script Extension page Background page
Simpler tabs.create option.
If related = true && relatedopenpos = 'related' then open a new tab with some URL as if that URL had been middle clicked on the current tab. If relatedopenpos = 'next', open it as the next tab. If 'last', open it last and don't tell Firefox who opened it.
Similarly for tabopenpos, but only tell FF that the newtab is related to the activeTab if tabopenpos == 'related'.
i.e. place that tab just after the current tab and set openerTabId
The first active tab in the currentWindow.
TODO: Highlander theory: Can there ever be more than one?