Thanks for posting your issue and resolution here. In the event you still have this data or can reproduce the issue, would you be able to post the full request and response headers here related to this issue? (feel free to remove any application-sensitive information) Also, what was the exact FF version number if you have it?
I ask only because FF 44 is the most recent at the time of this writing and errors that may be browser-specific might be helpful to others ife they are known issues.