Screen Grab will be very useful. It is easy enough to take screenshots, past them into GIMP and save them as PNG. Why do that when you can have something do it for you in one step?
As the name suggests, Screen Grab saves the entire webpage as an image. The screengrab plugin can capture the current browser window, any visible portion of the browser window and even capture the entire website being viewed in Firefox.
When you capture the whole web document, this screen capture plugin scrolls the page around taking snapshots every time. At the end it stitches them all back together again and asks you where you’d like to save the image. The screenshots are saved as PNG format. Screengrab requires Java Virtual Machine. Supports Firefox 1.0+