Is it legal to still hit the racquetball after it hits the front wall as long as it has only bounced on the floor once?
Yes, until the ball bounces twice on the floor, it is still in play - with the obvious exceptions like when the ball hits a player or goes out of the court.
I believe the scenario you're specifically referring to is when your opponent hits a hard, high shot off the front wall that then hits the back wall, bounces once on the ground and makes it all the way back to the front wall again before the second bounce. In this case, yes, you can still hit the ball as it rebounds off the front wall but before it hits the ground.
According to the 2013 USAR rulebook Rule 3.13 paragraph (e):
(e) Return Attempts. The ball remains in play until it touches the floor a second time; regardless of how many walls it makes contact with – including the front wall.