I followed instructions on this website, https://www.citrix.com/blogs/2017/03/15/firefox-52-and-citrix-receiver-for-web/. It seemed to work for a short while, but now it no longer works. If I use Chrome or Internet Explorer on the same system (using the same version of Citrix Receiver) i have no issues. Only the most recent versions of Firefox. I've been on the phone with Citrix Support and they have basically told me to reach out to Mozilla support on this issue.