Extensions are add-ons for the Firefox browser that add new functionality. There are several categories of extensions including themes, plugins, and dictionaries. Extensions can block ads, download videos, integrate with social media, and add features from other apps. The document provides examples of useful extensions, such as PubMed and NLM extensions for accessing medical resources, and Firefox Sync for syncing bookmarks and passwords across devices. It also explains how to find, install, and learn more about extensions.
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