Firefox 4.0 RC1 Fully Compatible Collection. (a list of mine.)
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
Fight the text ads! Element Hiding Helper is a companion extension for Adblock Plus meant to make creating element hiding rules easier.
Displays a country flag depicting the location of the current website's server and provides a multitude of tools such as site safety checks, whois, translation, similar sites, validation, URL shortening, and more...
Tab Mix Plus enhances Firefox's tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more. It also includes a full-featured session manager.
Personas are free, easy-to-install "skins" for Firefox. Personas Plus extends that built-in functionality to give you even more control and easier access to new, popular, and your own favorite Personas.
The One; The Original Fasterfox Gives performance & network tweaks for Firefox. Fasterfox makes your browsing so much faster!
Make your Sites Faster than a Fox with Fasterfox for FireFox!
Notifier for gmail, yahoo, hotmail, aol and more webmails.
X-notifier(aka WebMail Notifier) checks your webmail accounts and notifies the number of unread emails...
Supports : gmail, yahoo, hotmail, POP3/IMAP, facebook, twitter and more
Allows the download manager to also open in a tab or sidebar, and adds some optional display changes.
Status bar widgets and progress indicators for Firefox 4+
The extension previously known as GooglePreview. Inserts preview images (thumbnails) and popularity ranks of web sites into the Google, Yahoo, Bing and DuckDuckGo search results pages.
Make your bookmarks bar better. Roomy Bookmarks Toolbar can: hide bookmarks/folders name(dock like), multirow, autohide and many another features(more than 20).
Customize your bookmarks bar using one add-on.
Tweak Network will speed up the loading of web pages and increase the maximum number of simultaneous downloads from a site.
Set Default FullZoom Level,
Toolbar buttons for Page zoom and Text zoom.