For USPS or FedDex or UPS, I do recommended their smartphone apps this is really handy to keep track of your delivery. As to the site can you post a screenshot of the error? Also are you login to the site or just guest user?

I'm also having this problem as of February 4 2022: https://tools.usps.com/rcas.htm

It just gets stuck during initialization.

firefox does not work on usps scheduler . https://tools.usps.com/rcas.htm works great with chrome browser

What's not working? Can we get more info please? Did you try with Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode