Firefox Add-ons へようこそ。
On install, a toolbar button will be added to your browser (which you can customize) There's also a item on the right-click context-menu. Use any of them to activate the capturing process. You know, click and drag to select the area to capture - then, as soon you leave the mouse button it'll be automatically sent to the Firefox's PrintPreview window.
Version 0.1.5 introduces a new feature which allows you to save the captured area on the clipboard so that you can paste it anywhere. To do so you need to keep the CTRL key pressed before leaving the mouse button at capturing stage.
On Version 0.1.7+, using either MMB or RMB on the toolbar button will save on the clipboard the whole page or just the visible area, respectively. The ToolTip will remind you the actions.
(MMB = middle-mouse-button, RMB = right-mouse-button)