It might be pretty late, but you CAN change it in firefox (as well as in forks). I just put this in my user.js

user_pref("intl.date_time.pattern_override.connector_short", "{1} {0}");
user_pref("intl.date_time.pattern_override.date_short", "yyyy-MM-dd");
user_pref("intl.date_time.pattern_override.time_short", "HH:mm");

In gnome there’s dconf

Personally, I think methanation may be promising, since you can use the existing infrastructure.

Slugs do not work that way! Good night!

They asked me how well I understood theoretical physics.

I said I had a theoretical degree in physics.

They said welcome aboard.