Plugins/add-ons help your browser perform specific functions like viewing special graphic formats or playing multimedia files. Plugins are slightly different from extensions, which modify or add to existing functionality.

Here is a list of some best and useful firefox plugins/add-ons.

1. Firebug : Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
2. Adsense Notifier : Displays your Adsense earnings on the statusbar.
3. FireFTP : FireFTP is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers.