NOTE: There is this PR open for a time zone feature
Firefox's new setting to resist fingerprinting sets the time zone to UTC.
Steps to reproduce
In Firefox's about:config enable privacy.resistFingerprinting.
Restart the browser.
Open Mattermost and send some messages.
Either times are correct or the time zone is configurable.
Observed behavior (that appears unintentional)
Times are displayed in UTC.
Show a warning if the detected time zone is UTC. Add a user setting to override the time zone.