After more investigation and travelling back and forth, it appears the issue had at least 2 roots. One is that the update process somehow broke my existing profile. A new profile solves that. The second is that the latest version of firefox seems to not be able to authenticate with an NTLM proxy specifically for web.whatsapp.com.
This has happened before - https://bugzilla.mozilla.org/show_bug.cgi?id=1536787 shows almost identical behaviour to what I am experiencing. However I was not using Firefox at that time. Perhaps I'll just switch back to Chromium for whatsapp web.
