Playground no longer works for me, the window stays gray. I'm not exactly sure when and why this happened, maybe it is due to tightened security in some Firefox update in the last week or so. In the console I set the following security error:
I use the same link https://play.rust-lang.org/ .
The .js links above occurred in the error message. I have no idea what they do, looks like some obfuscated code to me.
It turned out that cookies were the culprit.
Some change in playground causes cookies to be set, and by default that is forbidden in my Firefox.
Cookies are a privacy risk, and it is a nuisance to allow them, so please use them only when strictly necessary.
As mentioned on the issue, the playground does not use cookies, but it does use local and session storage. Firefox (and maybe other browsers) treat local storage and session storage differently when very locked down. Specifically, window.localStorage exists, but calling methods on it will fail. window.sessionStorage straight up doesn't exist at all.
The recent change that added session storage additionally stores the code and last-chosen build mode in session storage, allowing for per-tab code storage.