Just out of interest I looked at Windy.com on Firefox/Linux and it looks fine? No blue/yellow banding. Wind gusts seem to show up fine. I've also had a look at the other stuff (e.g. waves, clouds, etc) which all seem to work as expected. This is with Firefox 78 LTS. Unless things have regressed in between version 78 to 90, I wonder if there is something else causing your issues?
So it seems like it was caused by FF blocking HTML5 canvas fingerprinting. Enabling that allowed it all to load correctly. I guess I must've hit disallow without thinking when I first visited, and that response was saved even after clearing cache, so that example at least is probably my fault.
I do wonder if every user would be aware that the 'Allow site to extract canvas data?' popup can break site functionality though -- its not as though there's a warning to that effect in browser.
And I have had a few experiences with sites not loading properly in the last few months. Try windy.com for example: on FF 90 with no extensions enabled and tracking protection on 'balanced' I get blue and yellow banding on the map, and the wind gusts graphics fail to load properly. I have reported a few of these instances in the past, but a lot of the time I'd rather just switch to chromium temporarily and continue with what I'm doing.
[0] https://www.cnet.com/tech/services-and-software/mozilla-exec...