...i'm going to replace them now or in the future.
You probably are going to replace them in the future.
Here are a few serious thoughts, though...
1) I can't recall ever having seen a firewire case made in the past decade that didn't also have a USB port. Your solution might be as simple as a cable change.
2) If your hard drives really are that old, the disk mechanisms inside them are probably at real risk of failure and should be replaced anyway. Old hard drives don't hold much data by modern standards. You could probably buy just one new drive with USB 3 that could hold ALL of the data from your previous drives.
3) If you absolutely want to keep the older drives, you could buy new external USB 3 cases for them.
4) If you don't want to buy any new hardware at all, keep them attached to your old Mac as a file server and connect your new Mac to them over your wired or wireless network.