Peacock no longer works. I can sign in, but the main menu appears for a split second, then the screen goes black.

Might be just some stale session data accumulated over time, try e.g. a Private Window to get a clean start to see if it's still the same, and then can move forwards with cleaning caches, cookies and site data selectively. I'd start with running Troubleshooting Mode first: [[Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems]] and if it's still broken there's steps how to report webcompat details with [[How do I report a broken site in Firefox desktop?]]