If it works with Win 8.1, it should work with Win 10.
MS released a signed legacy Firewire driver for win8/8.1 after 18 months or so of user complaints. I have used this after upgrading my PC and installing win 8.1. I suggest using this rather than the Win 7 Firewire driver if the provided Win 10 Firewire driver does not work.
You can D/L it here.
https://support.microsoft.com/en-us/kb/2970191If you use the W7 firewire driver in W8.1 x64, you have to run Win 8.1 in the "Driver evaluation mode" since the W7 is not properly signed . This is not necessary with the newer Win 8.1 driver.
However, I read somewhere (Cubase forum ?) that Win 10 contains the legacy driver AND it works with FW-1884.
As far as the Tascam/Sonar driver(s), with win 8.1, I had to install in compatability mode for W7 and I assume this will be necessary for W10 as well.
I have a new SSD intended for dual boot win10/7 on my old studio PC just for evaluation. I just need to download and install. If all goes well, I can move that box back to the studio and give it a try with the FW-1884. But like Bapu, that may not happen until the holiday.
If I did not need a Win 10 PC for work purposes, I am not sure that I would upgrade any of my PCs (2 desktops, 3 laptops on W8.1) until I absolutely must. I am very happy with Win 8.1 and unless I see some performance improvement with Sonar/Cubase on Win 10, I would probably stick with 8.1 (on my studio PC at least).