Cross-Origin Resource Sharing

Basically, it's a browser feature that prevents access to third party sites unless the third party explicitly opts in. This helps prevent certain types of XSS attacks, crypto-mining attacks, DOS attacks, etc.

Essentially, let's say you load epicgames.com, and the page tries to access steampowered.com for some reason. The browser then asks steampowered.com if it's allowed to access it from epicgames.com, and if not, it blocks the request. This means steampowered.com needs explicit support or that browser will bail.

CORS isn't super complicated, but browser implementations can vary so if a page doesn't make the request properly or the server isn't configured correctly, the browser will bail on the request. That's what's happening to Firefox, and usually it's the third party server that's at fault.