A Mozilla extension is an installable enhancement to the Mozilla browser that provides additional functionality (for example Linky, which adds an item to the context menu for opening multiple links in a document or selection).

They can add anything from a toolbar button to a completely new feature. They allow the application to be customized to fit the personal needs of each user if they need additional features, while keeping the applications small to download.

In Add on site for mozilla extension, I find that

Extensions are small add-ons that add new functionality to Firefox, from a simple toolbar button to a completely new feature. They allow you to customize Firefox to fit your own needs and preferences, while letting us keep Firefox itself light and lean.

Below, you can get more popular extension that you can download from mozilla.

Adblock Plus

VideoDownloader

FlashGot

NoScript

Download Statusbar

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