I'm not using a VPN or proxy. I do have uBlock Origin installed, but disabling it doesn't have any affect.
I've attached a HAR file containing the HTML and XHR responses to accessing the sign in page and a login request - the login returns a 403
That Zendesk article doesn't refer to Firefox or X-Frame-Options, so I don't see how it's relevant. Any content on the Plesk Help Center that requires sign-in is inaccessible at present in Firefox because of the inability to log in
This is still broken - I cannot log in using Firefox Developer 84: The loading of “https://plesk.zendesk.com/access/login” in a frame is denied by “X-Frame-Options“ directive set to “SAMEORIGIN“