Compare major firefox versions
WebExt code can be run from three contexts:
Content script Extension page Background page
await a promise and console.error and rethrow if it errors
Errors from promises don't get logged unless you seek them out.
There's an event for catching these, but it's not implemented in firefox yet: https://bugzilla.mozilla.org/show_bug.cgi?id=1269371
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?