Snapper is a Firefox extension developed by Joe Miller that adds the most common screen capture options you may need.
It adds a snapper icon to the status bar to start screen capturing. You then draw a rectangle over the web page to select the regions you want to capture. You can also resize and move the selection (must activate it through Options). Selections can then be copied to the clipboard, saved as JPEG or PNG files, or automatically saved as you capture them (great for batch capturing).
Note that if using the Anchored mode, where you can resize and move the selection, you have to double click the selection to save or copy it, which may not be obvious for everyone.
For advanced options like full page capture (with scrolling), annotation or timed capture, you may want to look for other options like FireShot. But if you need a quick way to share with others what you have in your screen, this is a great option.
Get Snapper at Mozilla Add-ons.