Connecting Firewire800 to Thunderbolt3

Maybe, will depend on the device and the Os your Mac is running.

You will need a collection of adapters to do this unfortunately, and it will not work on macOS Tahoe at all as it lacks support for firewire devices.

If you really want to try this you will ned a Thunderbolt 3/4/5 to Thunderbolt 2 Adapter, and then an Apple Thunderbolt 2 to Firewire adapter which will not be easy to get and will Riley have to be used from the likes of Ebay.

Thunderbolt 2 to Firewire Adapter - Ebay Search

Your Mac will need to be running macOS Sequoia or earlier.

Joe Baglione wrote:

Is it possible to use an older firewire devise with a newer iMac with Thunderbolt?

Firewire support was removed in macOS 26.x Tahoe. So you must use an older Mac which supports macOS 15.x Sequoia or older.

Edit: Sorry @Phil0124, somehow I missed the last line in your post about Sequoia. I think I didn't notice it due to being adjacent to the link. Didn't mean to repeat what you wrote.