Crewe made a lively start on an unusually pleasant May bank holiday evening in south Cheshire - and the visitors had to withstand an early bombardment.
But a flying dive to his left from Rovers keeper Thimothee Lo-Tutala to keep out Rio Adebisi’s header was as close as the hosts came.
And, as so often happens, goals change games.
Matt Craig got into space on the left side of the Crewe box in a beautifully worked move, cut the ball back and Molyneux’s first-time left-foot finish into the roof of the net was a stunner.
Matus Holocek did whistle a 25-yarder just over before the break as Crewe sought an equaliser.
But four minutes into the second half the Rovers lead was doubled.
Crewe made a mess of defending Rovers centre-half Richard Wood's header on the halfway line, that set Joe Ironside clear down the right and he squared for Biggins to drill home a low right-foot finish.
Whether it was the 50-yard dash to get on the end of it, or the dash to the touchline that followed to celebrate with the away fans, it proved the last of Biggins’ activity for the night, as he as was forced off, needing what looked like ice treatment on his hamstring.