When I view gallery images in the colorbox mode, they looks as expected in Chrome, IE, and Opera, while in Firefox (v.9.0.1) the colorbox looks downsized.
This is an example of the colorbox html code for a specific image viewed in most browsers:
<img width="540" height="405" typeof="foaf:Image" src="http://n1.localhost/files/styles/media_gallery_large/public/DSC00429.JPG" alt="">
And this is it for the same image viewed in Firefox:
<img height="405" width="540" alt="" src="http://n1.localhost/files/styles/media_gallery_large/public/DSC00429.JPG" typeof="foaf:Image" style="width: 250px; height: 188px;">
As you can see, there is an extra "style" attribute that somehow appears in Firefox and downsizes the colorbox image from 540x405 to 250x188.
(A strange thing is that this bug is not 100% reproducable: some images in a media gallery may look as expected, while other are downsized)