API bug? A user on Windows (20H2) Firefox is getting a static mono font when they request a variable MONO axis

Bringing this over from https://github.com/arrowtype/recursive/issues/460.

## Problem

@subnut is using a valid GF API URL to request Recursive with the `MONO` axis:

`https://fonts.googleapis.com/css2?family=Recursive:MONO@0..1&display=swap`

The URL they are using works fine for me in a test on CodePen, but the same CodePen only returns a static mono font for them. A screenshot from them:


If I download fonts from the font URLs in the CSS @subnut gets vs from the CSS I get, I do see that they are getting Monospace, static fonts, while I am getting fonts with a variable MONO axis.


## Version info

The fonts returned are Recursive v `1.047`. So, this is a problem from before this [week’s merged update of Recursive](https://github.com/google/fonts/pull/3279) (which probably hasn’t made it onto the GF servers quite yet).

## My best guess at what is going wrong

My best guess is, the Google Fonts API might be guessing wrong about what CSS to send @subnut’s OS/browser, and is probably sending CSS that would be sent to a system that couldn’t handle variable fonts. But, they are on a very recent version of Windows & Firefox, so that is surprising. Maybe Firefox is somehow masking their info, for privacy reasons?