Timezone stuck on UTC in Firefox

I have a weird issue in NixOS where the timezone detected by Firefox seems to be stuck on UTC. My system timezone is definitely set correctly and I see it in other applications and commands like date. But in Firefox, running Intl.DateTimeFormat().resolvedOptions().timeZone in the JS console shows UTC, and some web apps think my timezone is UTC. Some possibly relevant options in my config:

time.timeZone = "{my timezone}";
# needed for dual-boot with Windows so its clock is set correctly
time.hardwareClockInLocalTime = true;

It’s weird that only Firefox seems to have this issue. I found this thread which suggested the cause is having the privacy.resistFingerprinting option enabled in Firefox, but I currently have that disabled, and I couldn’t find any other Firefox settings that seemed relevant to timezones.
I’m using Firefox 94.0 from NixOS 21.05 stable.

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I got correct timezone on Firefox and nixos-21.05. You can look at my config GitHub - rofrol/nixfiles

And I have bios clock in UTC set like here System time - ArchWiki

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Yeah I was considering changing Windows’ clock as the alternative, but I heard it can cause issues in Windows. Have you had any problems with that?

No problems since couple of years.

FYI, toggling the privacy.resistFingerprinting flag changed Firefox from reporting UTC to reporting the OS time zone. Unfortunately there doesn’t seem to be an option to resist fingerprinting while allowing web sites to detect time zone.