If you wish to take screenshots of website, post or just a bit of piece that you see on website, you can use following listed firefox extension.
Both the extensions allow you to capture a part or full page of website. You can even annotate this screenshot and send it to your client , friends etc. I personally use Awesome Screenshot a lot to show web designs to clients. So next time, if you wish to take screenshots from Firefox browser, you know what to use 😉