December 15th is the earliest date that Mozilla is considering for actually stopping the Persona service, so we have at least a couple of weeks to solve this. I don't currently have a Windows VM handy, and won't have enough bandwidth to get one until this weekend. Thus, any help you can provide in trying to diagnose this failure would be immensely helpful.
Specifically, /confirm has an invisible HTML form that's configured to POST a proof of identity back to the website you're logging into (
https://github.com/portier/portier-broker/blob/master/tmpl/forward.mustache). JavaScript on that page should automatically submit the form, making it appear like a seamless redirect in most browsers (
https://github.com/portier/portier-broker/blob/master/res/static/forward.js). I suspect IE11 is, for some reason, refusing to submit the form on page load. Do you see any errors in the F12 developer tools' console?
If it's just refusing to run the JavaScript, we could make the form visible and require that IE11 users click a "continue" button to finish logging in...