Firewire Devices

There is the earlier FW400 (IEEE1394a) and later FW800 (IEEE1394B) specification. Incompatible plug architecture. The only FW800 adapter sold by Apple is for Macs equipped with a TB1 or TB2 port. It is supported on these Macs:

Apple also sells a USB-C Thunderbolt 3 to Thunderbolt 2 adapter that does not transmit power over the cable, so would not be appropriate for those FW800 devices that expect power from a cable connection when connecting these two cables. If your iMac has no TB3 port, then this is irrelevant.

There are FW400<->FW800 and FW800<->USB-A 2.0 dongles on Amazon, as well as FW400<->FW800 cables.

Stating the year of your iMac from Apple () menu > About This Mac would have been more helpful.

Yes there are, here’s one from the Apple Storehttps://www.apple.com/shop/product/MD464LL/A/apple-thunderbolt-to-firewire-adapter?fnode=9384f79d3a78dab896829df2038a5a3553c8105f07ba78ba627c803d960a63b7f01c6ca8cd6f36d83154728812ce341b95064569f1b791f7dc399313f20ed170a8a812ea6673cb4b40d0f0df91da6d4ed3eaf19c8f7e71ab27e60a1472b75990a549666537144e31bc38f54b3854c16b&fs=f%3Dadapter%26fh%3D4595%252B45b0