I've read that Thunderbird is supposed to use the system date settings but for me it uses the wrong locale for dates. As from the attached image, in my system settings the date is dd/mm/yyyy but thunderbird shows it as mm/dd/yyyy. On the Thunderbird preferences, it only shows en_US as an option. My environment has these variables: LC_ADDRESS=nl_NL.UTF-8 LC_NAME=nl_NL.UTF-8 LC_MONETARY=en_GB.UTF-8 LC_PAPER=nl_NL.UTF-8 LC_IDENTIFICATION=nl_NL.UTF-8 LC_TELEPHONE=nl_NL.UTF-8 LC_MEASUREMENT=gv_IM.UTF-8 LC_TIME=en_GB.UTF-8 LC_ALL=en_US.UTF-8 LC_NUMERIC=en_US.UTF-8 I'm using Thunderbird 78.5.0 on Kubuntu 20.10.