The Inspector now resolves var(), attr(), and env() in declaration steps, tracing them to their source. Calls to scheduler.yield() keep the task’s priority and abort signal across a synchronous await. Promise.try() passes native promises through, and using declarations can no longer be reassigned. Behind flags: named-feature() in @supports and the Container Timing API.
developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/156