Addons I cannot do without. I delayed migrating to FF4 because several of these addons were incompatible. History Sub Menus and Compact Menu2, I really hope that you make it to FF4 as Speed Dial did.
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).
LastPass, an award-winning password manager, saves your passwords and gives you secure access from every computer and mobile device.
Xmarks is the #1 bookmarking and open tab add-on. Keep your bookmarks, and open tabs backed up and synchronized across computers and browsers. Mobile phones with Premium access. Utilize LastPass to sync passwords!
Email This! will send your recipient the link, title, & highlighted text of the page you are viewing using GMail, Google Apps GMail, Yahoo, and Stand-Alone Mail Clients like Outlook Express, Thunderbird, & More!
Combined with the ChromeToPhone application for Android, this extension allows to send urls, text and files from Firefox to your phone or tablet.
Add a compact menu button to the navigation toolbar, and hide the menu bar.
KickassTorrents Toolbar is the ultimate toolbar for KickassTorrents that retrieves the latest torrents of KickassTorrents and adds easy to use KickassTorrents search capabilites into your browser.
This places each day's entire browsing history in a submenu of the history menu.