You use Firefox (or another browser) to view photos on Facebook, Picasa, and other photo-sharing sites, and with Slide Show Viewer, a Firefox extension, Windows users can use Firefox to view slideshows of the photos stored on their hard drive.
After installing the add-on (you can download it here), you'll find it located in the tools menu. You can find the tools menu by going to Option and clicking Menu Bar to show that toolbar. Then from Tools in the menu bar you'll see Slide Show Viewer listed. (You can also add an icon for Slide Show Viewer to the menu bar by following this path View > Toolbars > Customize and then dragging it from the list to the menu bar.)
Click Slide Show Viewer from the Tools menu and it opens in a new tab. From the menu in the upper-right corner of the page, you can navigate to a folder to view. In my experience, I could view only folders containing images. I couldn't, for example, view a folder of Office docs, though I'm not sure why anyone would want to watch a slideshow of Word and Excel docs.
To flip through pictures when viewing a slideshow, you can use the arrow keys (right or up arrow to move forward, left or down arrow to move back) or the mouse buttons (left button to move back and the right mouse button to move forward). You can use your mouse's scroll wheel to pan and scroll on full-size images that are too large to fit in your Firefox window.
Unfortunately, I couldn't find any way to play a slideshow where it would automatically flip through photos in a given folder. Also, I couldn't play a folder that contained folders of photos. For example, I could not view the Pictures folder but had to drill down to either the Sample Pictures or the Stage folder to view images.
From the control panel in the upper-right corner, you can use two check boxes to view the full image or have it cropped to fit your browser window. You can use the Escape key to hide and restore the control panel.