When security is enabled a token is needed access the service. The Referer is embedded in the token and used to authenicate. If the referer does not match the value embedded in the token, an "invalid token" error is returned. Since Firefox doesn't pass a referer it obviously won't match. This effectively means that there is no way to support FireFox when the application consumes a secure service and the required token is embedded in the client.
As a work around I use the ESRI proxy page to handle the server requests. This solution works because the request is server to server and avoids the client referer issue. The solution works well.
Now enter the SOE. You guessed it, a secured SOE. All of the ESRI samples I've seen of consuming SOE's in Silverlight are client side. This gets us back the referer problem.
Has anyone run into this? If so, were you able come up with a solution?