You can use
openssl asn1parse -in ffdhe4096.txt to decode the file into two numeric values, called
g (but those names are not encoded in the file, they are named this way in the Diffie Hellman protocol, see ANSI X9.42 which costs $100 but you can find pirated copies).
Mozilla's value is 4096-bit MODP Group from RFC 3526. The RFC has the formula that produces it, but not an explanation why that formula was chosen.
The ffdhe4096 value from RFC 7919 (which you must use if you use FFDHE with TLS 1.3, which nobody does) also has those "FF" at start and end.
The upper and lower 64 bits (8 bytes, 16 hex chars) are forced to 1, resulting in 16 "F" in the beginning and end of the value of
p. This is true for standard DH groups as far as I could find: Appendix E of RFC 2412 "The OAKLEY Key Determination Protocol" from November 1998 defines the so-called Oakley groups, long used for FFDHE, and provides an explanation for this choice:
The primes for groups 1 and 2 were selected to have certain
properties. The high order 64 bits are forced to 1. This helps the
classical remainder algorithm, because the trial quotient digit can
always be taken as the high order word of the dividend, possibly +1.
The low order 64 bits are forced to 1. This helps the Montgomery-
style remainder algorithms, because the multiplier digit can always
be taken to be the low order word of the dividend. The middle bits
are taken from the binary expansion of pi. This guarantees that they
are effectively random, while avoiding any suspicion that the primes
have secretly been selected to be weak.
Because both primes are based on pi, there is a large section of
overlap in the hexadecimal representations of the two primes. The
primes are chosen to be Sophie Germain primes (i.e., (P-1)/2 is also
prime), to have the maximum strength against the square-root attack
on the discrete logarithm problem.
The starting trial numbers were repeatedly incremented by 2^64 until
suitable primes were located.
Because these two primes are congruent to 7 (mod 8), 2 is a quadratic
residue of each prime. All powers of 2 will also be quadratic
residues. This prevents an opponent from learning the low order bit
of the Diffie-Hellman exponent (AKA the subgroup confinement
problem). Using 2 as a generator is efficient for some modular
exponentiation algorithms. [Note that 2 is technically not a
generator in the number theory sense, because it omits half of the
possible residues mod P. From a cryptographic viewpoint, this is a
Note that openssl dhparam has a
-check option, but it is too strict (demanding that if g is 2 then p mod 24 == 11), and though this value doesn't pass this test, it is safe to use. There is an answer about this on Crypto.SE.