From MozillaZine Knowledge Base
This preference controls part of the permissions extension, which is now built by default. The permissions extension allows for a more complete method of allowing, blocking, and restricting content displayed in the browser—including images. Here, the permissions preference takes over image behavior duties from network.image.imageBehavior. It works essentially the same way as the old preference, except the possible values are different (and in different order).
Exceptions to the image loading preference are stored in hostperm.1, as always.
Possible values and their effects
Allow all images to load, regardless of origin. (Default)
Block all images from loading.
Prevent third-party images from loading.
Still can't see images from third party sites
Even with permissions.default.image set to 1 you may still not be able to see images, particularly from third party sites.
Firefox 23 and later
In Firefox 23 and later you no longer have the "Load images automatically" setting in "Tools → Options → Content" to change the global image permission that controls this preference (see bug 851701 and bug 851698).
Deer Park and Mozilla Firefox 1.5
The checkboxes labeled “Load Images” and “for the originating web site only”, located under “Tools → Options → Content”, set this preference to 1 and 3, respectively.
The checkboxes labeled “Accept all images” and “Accept images that come from the originating server only”, located under “Edit → Preferences → Privacy & Security → Images” set this preference to 1 and 3, respectively, in SeaMonkey.
First checked in
Has an effect in