To start the image viewer right click on an image and select "View image" (the same can be done for videos supported natively by Firefox).
The box on the right is a demonstration of the ImageTweak viewer. Place the mouse pointer over it and zoom the image by using ctrl+mousewheel or ctrl+-/+. Rotate the image by using ctrl+alt+mousewheel or </>. Press 1 to have the image fitted to the box, 2 to have it fill the box.
To learn more about what you can do with ImageTweak, read the user guide. To customize your settings, edit your ImageTweak preferences.