https://searchfox.org/mozilla-central/rev/31f5847a4494b3646edabbdd7ea39cb88509afe2/mfbt/BloomFilter.h#138-140,142,154-158,167

static const size_t kArraySize = (1 << (KeySize - 3));
static const uint32_t kKeyMask = (1 << (KeySize - 3)) - 1;
static const uint32_t kKeyShift = 16;
...
static uint32_t hash1(uint32_t aHash) { return aHash & kKeyMask; }
...
void setSlot(uint32_t aHash) {
  uint32_t index = aHash / 8;
  uint8_t shift = aHash % 8;
  uint8_t bit = 1 << shift;
  mCounters[index] |= bit;
...
uint8_t mCounters[kArraySize];

KeySize - 3 in kArraySize is because single entry is uint8_t,

kKeyMask shouldn't use KeySize - 3 but just KeySize, because the result is used to point single bit, and the - 3 part is already performed by aHash / 8 and aHash % 8 in setSlot.

Depends on D175722

The point of a bloom filter is not to be correct, as false positive are acceptable.
Thus is technically not invalid to ignore some of the bits.

Is there any other reasoning behind the "part 1" ?

(In reply to Nicolas B. Pierron [:nbp] from comment #3)

The point of a bloom filter is not to be correct, as false positive are acceptable.
Thus is technically not invalid to ignore some of the bits.

Is there any other reasoning behind the "part 1" ?

the code was using 2 ** (KeySize - 3) bits, instead of 2 ** KeySize bits, which means the only 1/8 of the array was used.

https://hg.mozilla.org/mozilla-central/rev/932f5df42bd2
https://hg.mozilla.org/mozilla-central/rev/fd4414470dad