I was able to reproduce the issue in Firefox and confirmed this issue was with the browser not with SP or IDP federtion servres Technically the hit is not even reaching out to our servers, however I see post in the trace file. Seems like the browser is encoding the post XML which is misleading the post address to go somewhere else instead of hitting our fed servers. If the request is sent from Chrome or IE, everything is fine. Why is this happening? Is Firefox encoding the post messages differently from IE and Chrome? Interestingly Firefox in Mac seems to be working. I have made few changes to Firefox browser URL encoding logic but no luck. Issue stared from last 2 weeks. Before that it is working fine. Our main browserfor all testings is firefox. This issue is stopping us with our validations. your help is very much appreciated.