Strange, the difficulty of 1Firo is significantly lower than that of 1Sivo - why?
The change happens at a particular address:
Prefix difficulty: 77178 1QLa
Prefix difficulty: 78362 1QLb
Prefix difficulty: 4553521 1QLc
This is a quirk of how the 25-byte (50 digit hexadecimal) Bitcoin address is converted into Base58 (represented by numbers and letters), and the different maximum values that can be stored in 25 base256 digits vs 34 base58 digits.
The Bitcoin address in it's native binary form (that you never see) is 25 bytes, it's parts are:
byte 0: Network ID Byte (0x00 for main bitcoin network)
byte 1-20: ripemd160 hash (20 bytes) of sha256 hash (32 bytes) of 0x04+public key (65 bytes)
byte 21-24: checksum: first four bytes of sha256 hash of sha256 hash of bytes 0-20 above
This would be 50 hexadecimal characters long (base16), with a possible digit value between 0-F.
We will ignore byte 0, it is always 0x00, and Base58 conversion always preserves this leading 0 byte by directly encoding it as a "1".
That means the "vanity" part of the address is bytes 1-24, 24 bytes of random hash output, or 48 hexadecimal digits. The maximum value that this can be is 0x FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF ffff ffff (the lower case "f"s are the checksum, and won't be all FFFFs for the maximum ripemd160 value.)
We can guess from the output below that this maximum possible address value encoded in Base58 becomes 1QLbxxxxxx.... something:
See how the 1QLa
addresses are full length, but the 1QLc
addresses are one digit shorter? That is because the only way to have an address starting at 1QLc or greater is by having a binary address that is 59x smaller.