Firefox's Developer Toolbar features a `screenshot` command out of the box. Just hit Shift + F2 and type the command: https://developer.mozilla.org/en-US/docs/Tools/GCLI#Commands . Also, Firefox >=32 has a button for taking a full-page screenshot in the DevTools: https://developer.mozilla.org/en-US/docs/Tools/Tools_Toolbox...

Alternatively, for the use cases not needing to be in-browser, why restricting to extensions? Your platform has full featured apps doing the job out of the browser:

- Linux: I use http://shutter-project.org/

- Windows: I use (payware, but worth it, it does a lot) http://www.faststone.org/FSCaptureDetail.htm , or (foss) http://getgreenshot.org/