DownTheMall – download manager
Feedly, excellent news feed aggregator
NoScript, allows control over which scripts are executed on a per website basis, excellent for privacy and security control.
AdBlock Plus, blocks image and flash ads from pages you visit, fully configurable.
Firebug, web development tools.
WoT – Web of Trust, reputation based security tool.
ScribeFire, browser based blogging tool.
Lastpass – secure password manager
and some recommended bookmarlets for all browsers:
toPrintFriendly / toPrintliminator