1. Tools

Adblock Plus – Blocks advertisements.

NoScript – Allows JavaScript, Java and other executable content to run only from trusted domains.

X-notifier – Notifier for gmail, yahoo, hotmail, aol webmails.

iMacros – Automate Firefox. Record and replay repetitious work.

2. Interface

Bookmarks Sidebar Scroll Retainer – Memorizes Bookmarks/History sidebar’s vertical scroll position.

Add to Search Bar – Make any pages’ search functionality available in the Search Bar.

SearchWP – Search multiple words. Jump-to-word buttons and highlighting for the search box content

UrlbarExt – Extends the Location Bar with set of commands (Shorten URL, Copy URL, Search site, Go up, pNg, Tag pages easily, Navigate through sequential URLs, View page cached version, Surf anonymously using online servers, Twitter)

Downloads Window – Separate Downloads window.

3. Web Development

Firebug – Web Development toolbox.

Web Developer – The Toolbar with useful web tools.

DOM Inspector – Inspects the structure and properties of a window and its contents.

Element Inspector – In the DOM Inspector addon quickly navigates to elements by Shift+Rightclicking on it.

ColorZilla – Color picker.

Firesizer – Show window size in the addon bar on resizing. Set custom window size. Needs The Addon Bar addon.

4. Miscellaneous

Easy YouTube Video Downloader – Download YouTube videos.

Aniweather – Animated Weather.

The Addon Bar – restores the bar for F29+.

No Close Buttons – Remove the Close buttons from the tabs.