FIRM is still OK to use. In fact, the application still uses it (which you can see in the DSTasks logs). The advantage of FIRM is that it will FIND the production drive for you, where if you use Copy, you have to figure that out yourself. The disadvantage is that if it doesn't work, it's a pain to get around. I use copy when doing custom work and a quick script in VBS to find the production drive, but I use FIRM for any default imaging using the engine.
So feel free to keep using it.
HOWEVER
It is important to note that Replace Tokens and FIRM are 100% disconnected.
Example: FIRM is used in DS 7.1/7.5 but Replace Tokens is NOT. Well, sort of not.
Replace Tokens is executed by the engine on the server prior to sending any data to the client. FIRM is executed on the client using files given to it by the server, OR files you give it yourself. Replace Tokens modifies files anywhere using the engine to simply look through a file for known wild-cards, and doesn't have a clue if FIRM even exists. After using Replace Tokens, you can EITHER use FIRM, or Copy, or Robocopy, or VBS - whatever suits you.
And we all really miss Replace Tokens in DS 7.x. :(
Have fun!!