Following are the essential Firefox installation that I have installed on my system:

Essential Addons:

• AdBlock Plus: The quintessential Ad remover extensions!
• Aging Tabs: Allow you view your tabs by their active/inactive state (by changing color).
• Context Search: Allows your search the select text via context menu.
• CuteMenus - Crystal SVG: The essential addon for adding icons to Firefox menus!
• DictionarySearch: Search the selected word/phrase in a dictionary (via context menu).
• Flashgot: The quintessential download manager.
• Greasemonkey: The essential customization extension.
• Linkification: Linkification converts text links into genuine, clickable links.
• Paste And Go 3: It lets you Paste URL/Text and Load/Search it immediately ...
• Resizable Text AreaAllows your resize textarea box as per your choice.
• ScreenGrab: Screengrab! is an extension that makes it easy to save a web-page as an image - it's a screen capture tool for Firefox!
• TinyMenu: TinyMenu is an extension that allows you to reorganize the menu options so that you can maximize browsing space.

Addons for WebDeveloper:

• ColorZilla: Advanced Eyedropper, Color Picker, Palette Viewer and other colorful goodies for your Firefox.
• Firebug: One extension to debug them all!
• HTML Validator: HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon in the status bar when browsing.
• Live HTTP Headers: View HTTP headers of a page.
• Nightly Test Tools: This extension adds a few extras useful to those that regularly test nightly builds of Firefox, Thunderbird, Sunbird and Toolkit Seamonkey (Suiterunner)
• Web Developer: Adds a menu and a toolbar with various developer tools!

Addons for Social Networking:

• Delicious Bookmarks: The official Delicious extension.
• PowerTwitter: Makes twitter better (in Firefox)