As a Firefox user, how can I most smoothly transition to a new (to me) computer?

I know almost nothing about the physical workings and terminology of a desk top computer (appliance). It’s willful, as computers themselves don’t interest me, only what wonderful things they can accomplish.

I need help transitioning from an old Dell WinXP dying computer to a newer Dell Win7 (I think) model.

Nearly all my personal files are currently housed on, and accessed from, an external drive. The majority of that stuff should be plug-and-play. There’s plenty of room on the external, if I could copy-paste from the old PC to the external, and copy-paste to the new PC.

I have the equipment to hook both computers up simultaneously. I’ve transitioned that way previously, but it took a dedicated cable and program. If that’s what it takes, I can do that, but don’t see it as much more than a USB into each. Normally cheap and easy, I’m transitioning to easy and inexpensive. :-)

Thanks, be patient?