Try changing DNS over HTTPS (DoH) to "Off" or "Max Protection" and in Connection Settings change "Use system proxy settings" to "No proxy".

Does it still happen in Troubleshoot Mode?

I discovered the problem just after I posted. Troubleshoot mode did fix the problem, which usually means some extension is at fault. I had just added, at Mozilla's suggestion, Privacy Badger. I removed it, left Troubleshoot Mode, and everything works fine. Not sure why a "recommended" add-on would cause issues, but apparently it did. Appreciate your quick response, zeroknight! Thanks!