Comment by JdeBP
A quick test of Vivaldi with some of those shows it nowhere near as strict as your Firefox is. Amusingly
new Date("2025-07-12 12:30:45 BST")
is an invalid date, whereas all three of new Date("2025-07-12 12:30:45 GMT")
new Date("2025-07-12 12:30:45 UTC")
new Date("2025-07-12 12:30:45 Z")
are valid but in British Summer Time.