Thanks Ira, not just Mail, its' links on desktop, links I click from Drag Thing, super nuisance. Firefox is default browser for 10 years. No energy to change it. But the problem seemed to vanished on a 4 days test defaulting to Safari. I've posted on FF site, no one has a clue. Not many are with Mac over there.
It seems to happen after I quit FF (either intentionally or by hitting cmd-q when I meant to hit cmd-w). The first link click launches FF and brings up the blank page and only the second time does it launch the intended site.