QGIS does not automatically refresh the WFS layer unless feature caching is turned off. The selection box is so far right in the Add WFS layers that is tends to be hidden.
In a non-cached mode QGIS is sending a new WFS request after every pan or zoom by using the visible map area as a BBOX.
The refresh button did not work for you because in a cached mode WFS requests do not use BBOX. Resfresh brings you the same features again. However, if they were meanwhile edited in the database you would get fresh data.
