I have a Focusrite pro 24 DSP audio/MIDI interface unit, which uses Firewire connection, and a laptop with only USB input connectivity.
Can anyone recommend a suitable adapter/converter cable or device?
Google lists only a very few and these have almost entirely bad reviews.
Thanks.

I don't think I've heard of anyone successfully doing that.
 
I suggest you put the unit up on Ebay (If you haven't sold before, maybe a friend can do it?).  Or put an ad in your local reseller paper.  Recent Pro 24 DSPs have sold on Ebay for around $200. After the sale, you can put that money towards a new USB interface.
 
With USB interfaces as low as $79, it's just hard to justify spending any money on the adapter.

I agree, I could not imagine the nightmare of dealing with converting the data, talk about another layer of issues and latency. 
 
Keep the saffire for your desktop and pick up the USB interface that meets your needs for the laptop. 

yeah i looking into this a while ago to connect a sony handycam (fw) to a usb-only laptop, and came to the same conclusion (move on)

If your laptop has an  express card slot (is that the right word ?)
 you can use a firewire Express card for your laptop.
 
https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=firewire+Express+card
 
Never used one so that I cannot recommend an express card.
A TI chipset can´t be wrong , use that for my saffire pro 40 for almost 10 years on my dektop PC.

i have a fw mobo and proc i'll sell you.  Posting it on KVR.
 
https://www.kvraudio.com/....php?f=43&t=484033

Maarkr
i have a fw mobo and proc i'll sell you.  Posting it on KVR.
 
https://www.kvraudio.com/....php?f=43&t=484033

Won't do OP much good (He has a laptop). 

Thanks to all.
Seems Firewire to USB is a non-starter, even though Amazon include a Firewire-based ExpressCard with a USB-based ExpressCard reader as "Frequently bought together".
Caveat emptor.

Do they still have those card slots on laptops these days?