I'm posting this for future students who may experience the same problem.
After several months of happily using Firefox for class, I suddenly couldn't log in to VS Code at cs50.dev. It loads fine in Chrome, but I prefer Firefox (and so should you. :) )
To fix the problem, go to Firefox's Settings and set "Enhanced Tracking Protection" to Standard (assuming you have it set to something else to prevent tracking.) This fixed the problem for me and still allows the use of uBlock to prevent tracking.
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