There a number of standard verbs defined in HTTP:
GET- Retrieve a resource
POST- Submit new resource
PUT - update resource
DELETE - delete resource
When you enter a URL in a web browser it always uses the GET method, the method to retrieve the resource.
In your example Firefox is attempting to a do a GET on https://apex.oracle.com/pls/apex/orange/TestPOST
but you have not defined a GET handler for this resource, only a POST handler. ORDS then tells you that the GET method is not supported on the resource, which is the correct behaviour.
Thank you Colm, I have some reading to do