With bug 255123 fixed, beginning in tomorrow's nightly Windows builds you can
achieve the requested effect by setting Firefox to open external URLs in tabs
and setting the (UI-less) pref browser.tabs.loadDivertedInBackground true.
Arguably we're doubling up that pref and could employ YET another as well, but
there's just the one at this time.
With bug 255123 fixed, beginning in tomorrow's nightly Windows builds you can tabs.loadDivert edInBackground true.
achieve the requested effect by setting Firefox to open external URLs in tabs
and setting the (UI-less) pref browser.
Arguably we're doubling up that pref and could employ YET another as well, but
there's just the one at this time.