For those of you who have been using Firefox for some time now, you might have seen this behavior from Firefox. This usually happens when you open a link to an image file. When the image is too large to fit the Firefox browser display, it automatically shrinks the image to fit your window. You only have an option to zoom in to the image’s real size.

Here’s a way to control this:

1. Bring up your about:config page. If you still don’t know how, read how to do in-depth configuration.
2. In the “Filter” field type browser.enable_automatic_image_resizing
3. As you see, it’s a boolean type so double-click it to change its value to false
4. Close the about:config editor. The changes take effect immediately

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Source: Stop Automatic Image Resizing