This may have a really simple answer but why if the first 8 characters of the btc address are all unique do they need the other ones.
![Huh](https://bitcointalk.org/Smileys/default/huh.gif)
The first 8 characters are not "all unique", they just happen to be "unique enough" that it is extremely unlikely to ever have a collision.
Now Firstbits are different as they can be tied to a Bitcoin address because of the order they have been seen in the blockchain. For instance, the Firstbits "1" will uniquely identify the first Bitcoin address that starts with a 1 -- which happen to be the Bitcoin address for the generated coin in the genesis block. Every other Bitcoin address also starts with a 1, except the fist one in block 0 is the only one with a Firstbits of "1".