Gmail в Firefox показывает время в UTC, но не в хроме

I found it. The entry privacy.resistFingerprinting => true in about:config inform every website to use UTC.

There is no other way to use local time zone in Gmail without being tracked by fingerprinting services, which is an annoying bug arising from extra security, because the user may want to resist fingerprinting, but still use local time.

This answer is based on https://support.mozilla.org/en-US/questions/1212634