This isn't a home assistant thing, this is a Firefox thing. Firefox will use UTC to resist fingerprinting browser wide, meaning all sites, not just home assistant

there's a timezone setting in the user settings, but setting it to use the server timezone did not really help for me.

you can also add certain domains to the resistfingerprinting whitelist. I believe its done with the privacy.resistFingerprinting.exemptedDomains pref

Timezones are a pretty good identifing aspect to your browser so enabling privacy stuff often messes that up.

Yeah resistFingerprinting unfortunately breaks a lot of stuff.