You're confusing general purpose locks with special-purpose ones used in the memory allocator. Firefox has its own GP locks, you can find them here: https://sea

You're confusing general purpose locks with special-purpose ones used in the memory allocator. Firefox has its own GP locks, you can find them here: https://searchfox.org/mozilla-central/search?q=&path=xpcom%2Fthreads%2F*Mutex https://searchfox.org/mozilla-central/search?q=&path=xpcom%2...* However those are not suitable for use in the memory allocator, and neither are WebKit's. WebKit uses os_unfair_lock within its memory allocator: https://github.com/WebKit/WebKit/blob/520379e30f3b2b6d4de9957ab2afc97185879bc4/Source/bmalloc/libpas/src/libpas/pas_lock.h#L120 https://github.com/WebKit/WebKit/blob/520379e30f3b2b6d4de995... And so does Chromium: https://source.chromium.org/chromium/chromium/src/+/main:base/allocator/partition_allocator/spinning_mutex.h;l=54 https://source.chromium.org/chromium/chromium/src/+/main:bas...