Do you happen to be using color management by any chance? Believe it or not (because this does seem insane), disabling full color management fixed the site for me. That is, I changed the setting "gfx.color_management.mode" from 1 (manage all colors) back to 2, the default (which only adjusts the colors of images with tagged color profiles).

It's bonkers, but I think I understand what's happening here, actually. Relatively recently, Firefox started color correcting the content of <canvas> elements. However, some sites will download an image, load it into a canvas created in Javascript, and then use the data for some other purpose.

In particular, Windy downloads a bunch of what appears to be wind data in the form of false color images like [1][2]. Presumably they are using this data to generate the wind animation on the fly in the browser. However, if they're utilizing canvases as part of that process, (my copy of) Firefox is changing the colors out from under them as part of the color management process. Presumably a large enough change in the colors would be enough to break the wind display code completely.

[1] https://ims.windy.com/im/v3.0/forecast/ecmwf-hres/2021091012...

[2] https://i.imgur.com/z3hQKvs.jpg (backup of [1] in case the link dies)

Works fine for me in Firefox on Mac.