This bug is in mozjs(firefox) esr-60.

The function vixl::CPU::SetUp() is never called. As a result, on arm64 MozJS will clear instruction cache separately for each byte, greatly impacting JS performance. This is especially important on Graviton2 processors, where advanced kernels expose instruction cacheline size of a full page (usually, 4KB)

In firefox esr 68, the SetUp function is called from js::jit::InitProcessExecutableMemory.