The ‘load images automatically‘ option which used to exist in older versions of Firefox’s content settings, was in fact removed from version 23 and that option is no longer available for access in the current Firefox browser preferences. Worry not! You can still block all images loading in Firefox browser, this not only saves your bandwidth, but also speeds up your browsing experience -useful for users having slow connections, here is how that can be done.
Blocking Images on websites in Firefox browser
1. Visit about:config
2. Find permissions.default.image and change its value to 2, this disables the images.
Setting the above preference value to 3 lets the images on website to load, but blocks the third-party images. The default value is 1, which sets the Firefox browser to load images automatically.
You may also want to check Image Block extension which offers toggle button to allow/block loading of images on webpages.