Software that adds functionality to a Web browser. Typically from a third party rather than the browser developer and also called "plug-ins" and "add-ons," browser extensions are used to augment the browser's capability. See browser engine
, helper application
and plug-in free
This list on the Firefox browser extension site shows the breadth of add-ons offered. There is a huge number of extensions available for major browsers.
The 3CX Phone Extension
After installing the extension on the Chrome browser, clicking the 3CX icon on the toolbar opens this dialog to make an IP phone call.
Page Capture Extension
GoFullPage is a Chromium extension that captures the entire Web page. In this example, it creates three separate images. See Chromium