Using the built-in developer tools
- Right-click the image and select the "Inspect element" option (or press Q).
- If it doesn't open the element tree vew, then press the second icon on the toolbar at the bottom.
- Then simply delete the selected node by right-clicking it and selecting "Delete node" option.
You can refresh the page to restore it. (This works in most recent versions of Firefox without any extension, btw.)
Using a bookmarklet
If nothing helps, and you are not able to add extensions, too, drag this code to your bookmarks tab (it's a bookmarklet), and click it when you are on the right page.
The code goes over all images on the page, and removes their
src attribute. A bit heavy handed, but it should work.