From a privacy perspective i simply don't want websites to know my system timezone. If i use a VPN which i do all the time these days, any website i visit will know that the timezone from the IP does not match the timezone of my operatingsystem, so they can basically guess that im using a VPN or even guess my real location based on the operatingsystem's timezone... This sucks badly. You might ask why i care and why im so possessed with privacy, i am not. I have nothing to hide and still i don't want bigtech and governments to have access to my personal information, it simply is none of their bussines period.
Being able to change the timezone in firefox SHOULD be a basic setting.