Try changing your system locale to a region that has a more desirable short date format. For example, Ireland (en_IE) uses 24 hour DD/MM/YYYY and South Africa (en_ZA) uses 24 hour YYYY/MM/DD. You should see this reflected in "Regional Preferences" in about:support.

Thanks @zeroknight for replying. Found the many regional entries in FF about:support, but I'm unsure where each is to be set, and what each affects.

Selecting a system-wide region to something of what you suggest could probably mess up other applications on my PC, such as spelling check languages, time zone, number formats, daylight-saving dates, etc. But maybe also affect browser searches e.g. South-African local search results?

Windows regional settings does provide fine-grained control about date/time/number/currency formats, which all (most) programs use. Unfortunately, Mozilla/Firefox appears to have some biased ideas of what a certain region is supposed to use. Is that because of its USA roots?

Well, I do live in the Netherlands. So, my proper regional setting is "nl-NL" or just "nl". However, I like to have all my programs in English if I can choose. My keyboard is en-US (not US-international!). My number format is not NL but more en-US or en-GB, except currency (Euro) and Measurement system (metric).

My date format preference (yyyy-MM-dd) is because of sortability and readability, as in never guessing between US month-day-year or Euro day-month-year. At least, big numbers do we all write from most-significant to least-significant (except little-endian computers :-); why not for dates?