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What I meant is, when did they start the switch, and which versions have already switched.

Nightly is based off of mozilla-central, our trunk.

When Nightly switches, that means that the version currently being developed (64), is the first version that has been switched.

Fx63 (on Beta and Developer Edition) is on track to ship with clang-cl as the Windows compiler too.

Windows builds were switched to clang back in July before Linux. MacOS was already using it.