Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
Summary
The :-moz-user-disabled CSS pseudo-class matches elements representing images that were not loaded because images have been entirely disabled by the user's preferences.
This selector is intended for use mainly by theme developers.