Not supporting Safari is weird, but not supporting Firefox is perfectly normal, given that it's at a whopping 4.46% of market share in the US.
http://gs.statcounter.com/browser-market-share/desktop/north...
It wouldn be interesting to see an evening, past work hours, US/Canada browser split. I would assume IE would tank vs being in 2nd. Edge would probably go down too though not as much.