I recently changed my internet provider and since then I am unable to access the page myibay.com and to use the Firefox plugin. Nevertheless, I can connect to myibay.com using a proxy site like tunneldirect.com.
Do you have any suggestion why it would be blocked or how I could avoid it. Perhaps it is only a problem of the new router. The provider is ono.es in Spain

Did you try to contact your provider so they can check why is it blocked?
It does not sound like it is a router's problem.