ScreenGrab is a great tool. I use it all the time for my web marketing business, McBuzz Communications.
One suggestion: I didn't notice immediately that the ScreenGrab tool uses a context menu: you just right click on a web page in your Firefox browser and the ScreenGrab! option is at the bottom of the menu. (I assume there is a corresponding contextual menu on Firefox for Mac, but I haven't tried it.) I was looking in the Tool menu and the View > Toolbars menu for ScreenGrab, but there is nothing there.