Connected from France, Firefox in english, default was °F

Is that localisation EN-US or EN-GB?

It's °C for me with EN-GB

deg F,

windows: en-ca chrome: EN-US Physically in Canada

Chrome in English, from Uruguay, default was ºF

Same here.

Connected from Ireland, Firefox in English, default was °F

Possibly the locale detection is limited/imperfect, with °F as a fallback.

Using ºF is an odd fallback, considering it's just one country that uses it (just make sure detection works ok for it and you're done).

You can see the available locales in the setting panel.

Also Celsius for me, seems likely to be region/locale based (as it should be, I'd say).

Call me weird, but I hate when a program or service uses regions or locales to determine stuff like temperatures or languages.

First of all, it often does it wrong - especially when using geographical location, which is one of the dumbest idea I've seen in computing. GPS fails when people are traveling (just because I'm in Germany right now, doesn't mean I want to see websites in German, etc.). GeoIP fails for various reasons, including VPNs and weird ISP shenanigans.

Second of all, as a person fluent in English, I especially don't want to see a translation of your originally English site. Most software and website translations suck hard. My most common gripe: using the same word in translated language for things named by different words in original, or vice versa.

To generalize this: when programs try to be clever, they often wind up being stupid.

I'd personally prefer that programs just acknowledge they're stupid and give me, the user, more control.