I just discovered that Firefox supports the geo URI scheme: e.g. Brandenburg Gate (geo:52.516272,13.377722, perhaps SU's markdown doesn't support it). It flashes up a blink-and-you'll-miss-it message "Redirecting to your selected maps website...". For me, at least, that website seems to be qwant.com.
How can I select something else? I expected it to be configured alongside my search engine in the "Search" section, but I can't see anything there, or even in about:config.
It would be really useful for me to be able to change it to a custom website.