Connecting external HD to M1 Mini

Firewire to Thunderbolt Adapter from Apple, and a Thunderbolt 2 to Thunderbolt 3 (USB-C) adapter from Apple.

I would advise replacing your drives with newer SSDs (or HDDs if the capacity is above 2TB), because FireWire is quite obsolete in 2021.

What FireWire external drive?

Many of those older external drives had both FireWire and USB connections. If so, then all you need is find the supplied USB cable or a replacement USB cable.

Another option is to pull the Hard Drive out of the FireWire enclosure and plug it into a USB to SATA adapter.

You're welcome.

What model is that LaCie?

I'm asking because some have 2.5" drives and some have 3.5" drives

and because some are easier to crack open and remove the drive than others.

Plus a USB to SATA adapter is a lot cheaper, than those expensive Apple adapters that may only get used one or two times.