When I changed over from Firefox 3.6.13 to Firefox 4 Beta 11, the standard utility I use for ‘complete page’ screenshots, ScreenGrab, was not supported in Fx 4. However the alternative Screenshot Pimp is fully supported and works great.
If you need a utility to grab quick screenshots and save videos, Screenshot Pimp is highly recommended; it works great and to date is the best screenshot utility I’ve used for Firefox since ScreenGrab.
Link for Screenshot Pimp is below the video.
Link: Screenshot Pimp