For anyone having problems going to the site at the https://www.wincustomize.com/ URL in Firefox, you have to clear your cache entirely.  Make sure you are logged out of the site in Firefox and Internet Explorer, not logged at all.  Make sure you close both browsers, then open Firefox back up.  Next, you have to go into your Clear Recent History settings in Firefox, check ALL boxes to clear, and set the time range to clear to Everything in the drop down list.  Clear your cache, then close Firefox and restart it.  The URL in question should now work fine.

And 30 min later, it should be fixed. Cookie on your end seemed to be trying to set a theme that isn't present.