"A browser or device determines the orientation by listening to the width and height of the window. If the height is larger than the width the window is in portrait mode. If the width is larger than the height it’s in landscape mode."
More info: http://www.1stwebdesigner.com/css/how-to-use-css3-orientation-media-queries/
Basically you need to resize your browser window for the portait orientation css to take effect.
If you open firebug within the browser window ie. firebug is at the bottom, the height of the browser window changes, causing the orientation to become landscape, thus you are losing your portrait styles. For both firebug and chrome developer tools, try launching the tools in their own window so your browser size is not affected.
PS. I would use Firebug for this kind of editing because you can clearly see the css changing as you resize the browser.