LOOKING FOR FIREWIRE 800 TO USB 3.0 USB ADAPTER/CABLE

There is no such thing!

FireWire and USB are two different incompatible protocols for transferring data. There are no adapter cables that will do what you want to do.

IF the Seagate drive you have is USB 3.0, if a new Mac, use the USB 3.0 connections. USB 3.0 is much faster than FW800. If your Mac isn't new and has USB 2.0 ports, you are stuck using these. The speed of the drive will automatically be reduced to USB 2.0 speeds.

IF you need a FireWire drive, you should purchase one.

Let us all know if these work with your Seagate drives.

You never mentioned what type of Seagate external drives you were using.

Older style Seagate drives do not have that special long end connector for using the adapters you found and linked to.

Older style Seagate drive enclosures only have the normal type of standard USB connection.

Good Luck!

Search the Apple online store and do a Google search as well search the various Mac reseller stores.

I've never heard of an adapter that will connect FW to USB; take a look here:

https://discussions.apple.com/thread/4958409