6 Pin Firewire to Thunderbolt

Hi, I am way behind the times...smh!

Last year in 2016 I purchased a new iMac (Retina 5K, 27", late 2015 model). I didn't realize that the new iMacs no longer had firewire ports on them.

Well, I have an MBox Pro interface that connects via a 6-pin firewire cable (see image below). I am desperately trying to locate an adapter that will allow me to connect to my iMac.

My MBox Pro interface uses a 1394 iEEEE 6-Pin cable, and unfortunately none of the adapters that I've found online have the correct thunderbolt adapter end on them. My new iMac thunderbolt ports have the shape of a square

"D" kind of (see image below).

No adapter that I've seen has the 1394 iEEEE 6-pin connection on one end, and the "D" shaped male end to plug into my iMac on the other.

Can anyone advise me? Do you know of such an adapter? I appreciate your help!

Aaron