This article  taught you how to resize images in Firefox. That extension includes zoom options to rescale pictures. With the Image Resizer/Scaler add-on for Firefox you can instead rescale the images simply by dragging them.
Resize images by dragging them
To install Image Resizer, open the add-on’s page . Then press the + Add to Firefox and Install Now buttons on that page to add it to the browser.
Next, move the cursor over an image on a website page such as the one below. Then hold the left mouse button and drag the mouse left and right. Moving mouse to left will reduce the image, and dragging to the right expands the image. After rescaling image, press the right mouse button over a picture to restore it.
Press the Open menu button and Add-ons to open the extension page in Firefox. Press the Options button beside Image Resizer to open the page below. That includes a few extra options for you to select from.
There you can select the Hold CTRL to Enable check box. Then you press the CTRL key and drag to rescale images. Select the Prevent images from being resized below 32 pixels in size to ensure that images aren’t reduced by any less than 32 pixels.
Overall, this is a good alternative to some of the other image rescale extensions for Firefox. With it you can rescale images to enhance their clarity, but remember the add-on has little to do with enhancing picture quality.