Browser used was Firefox 79.0 64 Bit. Browser Extension 6.4.0
I did not test 6.5.0 in other browsers, but the non-beta release 6.4.2 worked across both Firefox 79 and Google Chrome 84. Log in issue was specific to 6.5.0, Firefox 79.0 and those three sites.
I have today, following your reply, updated Firefox to 80.0 and reinstalled 6.5.0 Beta and on testing it now works with the three sites in question. There may have been a glitch, but the latest version of Firefox 80.0 and Google Chrome 84.0 both appear to work correctly with 6.5.0 Beta.
Thank you for the investigation and hopefully it's sorted itself.