Home Assistant is open source home automation that puts local control and privacy first. Powered by a worldwide community of tinkerers and DIY enthusiasts. Perfect to run on a Raspberry Pi or a local server. Available for free at home-assistant.io
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.