I want to unblock picture viewing. I clicked on "block picture view'' and now I wish to undo that change. how do I do it?

this is a feature - new to me -- that allows you to block the image of an emailed picture that has just been received by your computer. after copying the image, I wanted it to go away -- and on rightclicking over the image, I saw "block picture view.'' I clicked it -- and the image disappeared. Then Firefox announced: Firefox will always block picture view from now on. I didn't want that -- but there was no option to undo it. No image comes up now with an emailed picture. Only a blank space -- and if I right click over it, there is no option regarding blocking or unblocking "picture view."