Trial install. Issues.

Firewire 800 = 800 M
USB 3.0 = 5 G = 5000 M

Have I been mistaken in thinking 5000 was faster than 800.

I think what you read about was USB 2.0 which is between them. USB 3.0 is way faster than those two rated Firewire speeds.

I must have read it wrong. 3.0 would kill firewire

USB 3.0's theoretical maximum speed is 500 megabytes per second. (Real speed is ~400.)
FireWire 800 is 100 megabytes per second.
FireWire 1600 is 200 megabytes per second.
FireWire 3200 is 400 megabytes per second.

USB won't kill FireWire, or vice versa. USB is popular for home computing because it's cheap and convenient. FireWire is everywhere on prosumer and professional products because it offers the full bandwidth listed on the box (USB doesn't), and FireWire doesn't use the CPU to handle I/O control, leaving the system to run at full speed under heavy transfers.