When you browse the web with Firefox and attempt to view an image file (a JPG, for example), Firefox wants to ensure you see the “whole” image and therefore resizes the image dynamically to ensure a complete view. Unfortunately, resizing the image often results in less-than-optimal quality, a situation that’s easily remedied in Firefox by simply clicking on the image once to view it at its original full-glory resolution.
While the ability to full-size images in this way is handy, some people will want to view all images in their full size by default. A simple Firefox tweak will get the job done if this is what you want:
In the Firefox address bar, type about:config and press Enter. Next, browse to the preference named browser.enable_automatic_image_resizing. This value is set to true by default, but if you double-click on it, it will switch to false. Once complete, all images will appear without any resizing applied bu default. To switch to the resized, fitted view, simply click on the image as you normally would.