Basics

• Firefox can dynamically scale an image to fit in your browser window, which means that any image you load will change its size in order to fit properly, even if you re-size or move the window. This option is typically enabled by default.
  
  

Uses

• This feature is helpful for viewing large images all at once. If you load an image that is very wide, tall or both, it can help you view the entire image without having to scroll back and forth or up and down. Of course, you also have the option to click anywhere on the image to zoom in to 100 percent in that area, and then scroll around to see the entire image in full size.
  
  

Enabling & Disabling Automatic Scaling

• To change automatic scaling in your browser, type "about:config" into your address bar, then press "Enter." If a warning comes up, click, "I'll be careful, I promise!" In the search bar at the top, paste the following text: "browser.enable_automatic_image_resizing". If the Status is set to "default" and the Value set to "true," it means auto-scaling is enabled, and your images will re-size automatically. To change this, double-click the Preference Name; the text will become bold, the Status will become "user set" and the Value will become "false." If it is already like this and you want to change it back to default, simply double-click it again.
  
  

Considerations

• If you don't feel comfortable messing with Firefox's config page, you can try to use an add-on to accomplish the same function and possibly get even more features. Search for add-ons such as ResizeIMG, ImageFit and Image Assistant.