A new class of timing attacks (Meltdown and Spectre) that work on modern CPUs. Firefox disabling or reducing precision of several time sources in Firefox as a partial, short-term, mitigation. The full extent of this class of attack is still under investigation and we are working with security researchers and other browser vendors to fully understand the threat and fixes. The resolution of performance.now() will be reduced to 20s. The SharedArrayBuffer feature is being disabled by default.”]
Source: https://blog.mozilla.org/security/2018/01/03/mitigations-landing-new-class-timing-attack/