The ability to take screenshots has been added in a recent build of the Firefox Nightly release. Users of Firefox Nightly will notice the new screen capture icon in the toolbar alongside the download and home buttons.
Currently, you can take two sorts of screenshots in Firefox, full page or selected area. The full page option is self-explanatory, and takes a capture of the entire web page that’s open, the selected area option greys out the web page and allows you to define which parts of the page you want to capture. Once captured, the image can be stored locally or shared on screenshots.firefox.com.
By backing up screenshots online - which is optional - you can easily pull up a gallery of your saved captures; it’s quite a useful feature to have if you ever need to archive captures for future reference. If you do upload screenshots online though, be aware that they’re posted to public URLs, meaning anyone with knowledge of the URL can access the image.
An extension for Chrome is also in the works. Mozilla’s move to WebExtensions should mean that there will be little effort needed to create an add-on for Google's browser. At the same time, the firm will be able to put the Firefox brand name in users' Chrome browsers, which might convince some to give Firefox a look.