Did longbows have to be put in the ground to be fired better?

No, the English longbow could be fired by one man, although after years of training and firing, their skeletal and muscle structure was all bent and skewed. the Crossbow had to be placed nose first onto the ground and wound with a spring to pull it back, so often there would be a two man team. One firing a crossbow, and the other reloading. 

The answer is promontory, utah.