Do you want to hide large images or merely make them smaller by setting a max-width and max-height?

You would probably have to use JavaScript (bookmarklet) to achieve this reliably. There are bookmarklets to make large images fit in the window to avoid a horizontal scroll bar

You can try this code in the userContent.css file (adjust the max-width accordingly).

• http://kb.mozillazine.org/userContent.css

img { max-width: 1024px !important; height: auto !important; }

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

• Create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• Use a plain text editor like Notepad to create a (new) userContent.css file in the chrome folder (file name is case sensitive)
• Paste the code in the userContent.css file in the editor window
• Make sure that you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userContent.css.
  Otherwise Windows may add a hidden .txt file extension and you end up with a not working userContent.css.txt file

Yes. This worked. Thank you very much. There is just that even if the images is resized, the frameholder/space assigned for the image is still taking up the same space as the original image, but this will have to be better than nothing. Thank you.

UPDATE 18th april 2015. I updated to Firefox v38, and now the images are large again. The above soulution does not work on FF38. Can anyone help me, again?