post on the ubbthreads forum states, this error is caused by the Referer header not being recieved by the forum software.
As this is only happening in Firefox it's likely you've disabled the sending of referers in about:config (a setting which will persist in Safe Mode). Type about:config in the FF address bar and ensure network.http.sendRefererHeader is set to 2 (the default). Note that some security software blocks referers and it's not inconceivable that it would decide to do so only for Firefox.
I was able to reproduce this problem by setting network.http.sendRefererHeader to 0.