Two 100% identical Umbraco 7.5.2 webs are running on a Windows 2008R2 server with IIS 7.5 and a Windows 2012 server with IIS 8.5. Version .NET is 4.5.2.
Member login using method this.Members.Login in Umbraco SurfaceController works in all browsers - just in Firefox 64 it only works in IIS 7.5. Members.Login seems to return true but no Forms authentication cookie is set.
We tested 32bit vs 64bit, various random IIS settings - everything is set identically. No idea - we are going crazy.
Our guess it might have something to do with FF Cookie handling on POST with a 302 redirect afterwards. But here, too, browser dev tools show identical stuff.
Any idea about any applicable differences regarding Firefox/IIS 7.5-8.5/POST/redirect is highly appreciated.
UPDATE: it is a cookie issue - in some Firefoxes the ASP.NET MVC authentication cookie is not set.
The solution was stupid, but I put this here in case somebody has a similar experience.
The server was set one hour too early, and Firefox 64 doesn't allow Cookies that are set with a time like that. All other browsers do.