Well right now there is only one hidden feature. Be aware that this feature is hidden for a reason. The reason being that this feature may slow down your browsing experience on certain sites containing a lot a pictures. If you wish to use it that is fine but just be aware that if your browser slows down it is most probably due to this feature.
OK, with that out of the way, the hidden feature will autofit any images larger than your browser screen. This works well on some sites but not so well on others. This is particularly useful in Thunderbird when someone sends you a huge photo, this feature will automatically shrink the photo to the size of your client screen. To enable this feature, you need to set the preference in about:config for "imagezoom.showAutoFitInMenu" to true. A new option will then appear in your View->Image Zoom menu to "Autofit Images". This feature is not officially supported at this stage so don't write to me and say that its broken.
happy birthday meng