@sam feels quite important to improve this bit!
-> remove optional showing, or at least have it on by default
-> have start and end time in the same time zone (I mean, do people ever want those in different time zones? )
-> have UTC timezones right at the top of the drop down
.... etc
found this issue about improving the thunderbird calendar timezones picker. 13 years old, but active again since last year
https://bugzilla.mozilla.org/show_bug.cgi?id=647632
big changes coming apparently: "dialogs and cramped pieces of UI will be nuked after 115"