It is not recommended. Here’s why:
Let’s say you copy a 1 GB file from your computer to 2 external hard drives. One is USB 2 and one is firewire.
Both copies will take about the same amount of time, but the way it is done is different. The firewire drive will transfer the data at a nice, even rate. The USB drive will sometimes go much faster, but sometimes much slower. Over time, it all evens out and the transfer time ends up being very similar.
So for normal computer use, either type of drive is fine. Lots of people use USB 2 drives and are perfectly happy with them.
But for video, it’s a different story. We’re not concerned with “average over time” when it comes to copying video off of a tape. We’re concerned with x frames per second and maintaining that number no matter what. USB 2 is a bad choice because those dips in the transfer rate lead to dropped frames. Firewire might not hit the same peak rates as USB does, but it will maintain a usable data rate at all times which is what matters most when you’re editing.