Most likely Ad Block Plus. Look for the stop-sign like icon for the program on Firefox; to the right of the icon is a small arrow, click it and select "Open Blockable Items". A window should open in the bottom of Firefox which displays a HTML link for every item on the web page. You can do mouse-overs of the images, and when you find one you need to restore just right-click it and select "Disable Filter on "whateverlinkname". This will allow it to be displayed on the screen again.
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Shaowstrike (Retired - FFXI)
91PUP/BLM 86SMN/BST 76DRK
Cooking/Fishing 100 "We don't just borrow words; on occasion, English has pursued other languages down alleyways to beat them unconscious and rifle their pockets for new vocabulary."
— James D. Nicoll