The value of the firm can be assumed to be the present value of future cash flows
To me, this sentence makes no assumption. It states something that is true, whether there are future cash flows or not. It would be synonymous with:
The value of the firm can be assumed to be the present value of any future cash flows
The value of the firm can be assumed to be the present value of the future cash flows
On the other hand, this one seems to assume there will be future cash flows; that future cash flows have already been discussed and are being brought up again; or that there is some comparison to another type of cash flows* (past, for example). However, if there is no expectation or mention of cash flows occurring in the future, using the here is a bit awkward.
* - Thanks to @Tiercelet's comment about this.