Just to clarify...are you looking for an eraser to be able to erase the annotations you add with the pdf reader itself? Or an eraser for any content of the pdf?
The first is already possible—just click on the added annotations to select them and press CANC (or right click and delete). But the second part is not possible.
If you're on the viewer of course you can't edit the document, you have to save it and then you can open it to edit it. When you're viewing something you're not supposed to be able to edit it.
Replying to my own mssg! . . Just thought of a bodge . . select a free draw pen big enough and using background colour to overwrite the mistake? . . but surely Mozilla can supply a better eraser?? 😉