Plug old Firewire DV camera into USB using Firewire/USB adapter?

The obvious solution is the one you already mentioned: Thunderbolt-Firewire adapter. Or you can import to an external drive connected to one of the older macs and then attach this drive to the new mac. Or you can copy the files over your local network (which will not be that bad if you have the machines wired to Gb Ethernet).

All of the above are known to work without fuss. Usb-to-Firewire adapter? Never heard of any. If you can find one it may not be cheaper than the Thunderbolt adapter-that is assuming such usb-fw exists. Plus it may be plagued with driver problems since it is likely old and not made for current os... seems to me that it might just be a bag of hurt for no benefit.

Have a look at this:

OWC 13-Port Thunderbolt 3 Dock - Silver at MacSales.com

I'd jump for using a dedicated Mac with FireWire or thunderbolt with FireWire adapter.

I know the later will work fine.

Older USB 2 & 3 is a dead end.

The newer USB-C ports run into trouble with it's port power supply that's why the above mentioned Dock would be a possibility.

Al

I would not worry too much about your old macs. Yes, they will someday die. But your Firewire camera may well die before them. And your tapes, well... it is a good idea to extract your valuable videos from the past and bring them to the present.

Some two or three years ago, I copied all my old Digital8 tapes (and some older analog Hi8 tapes) to a mac, converting to H264 in the process. These are memories of my young children that are immensely valuable to me, but not the cameras I used to film them. They were first analog, and then digital but recording in SD in DV format, and I would not want to use those cameras anymore. The quality that amazed me 16 years ago is very poor today.

Those cameras were, in different moments in time, handed down to family members.

Quicktime Player doesn't play DV material without converting, and soon may not play it at all.