Privacy

Adblock Plus
Block advertisements
BugMeNot
Login using shared accounts
ublock origin
Block unwanted third-party scripts and cookies
privacy badger
Protects your privacy
Ghostery
Anti-tracking
ShareMeNot
Removes social sharing buttons and prevents their cookies
HTTPS Everywhere
Enables HTTPS encryption automatically on sites that are known to support it, even when you type URLs or follow links that omit the https: prefix.

Security

Tree Style Tab
Tabs on the left
fox input
Ctrl + i cycles focus through the page's text input boxes.
Classic Compact
Allows compact UI with advanced settings.
Classic Theme Restorer
Squared tabs, appmenu, add-on bar, small button view and many more 'old' features for Firefox 29+ (Windows / MacOSX / Linux).
FLST
Switch between current and last selected tab with click or Ctrl + .
SmoothWheel
Scroll smoothly
Show Profile
Displays the name of the Firefox profile on window title or customized locations. Only useful if you have multiple Firefox profiles (I have many. See why here.).
ProCon Latte Content Filter
Simple whitelist / blacklist site filter. Not really secure, but can help prevent you from browsing sites you don't want to browse within a specific browser profile.
Stylish
Lets you easily install themes / CSS modifications for sites.
Restartless Restart
Adds a quick method (File->Restart) to restart the browser.
Nuke Anything Enhanced
Allows you to hide almost anything via context menu 'Remove Object'. Great when you just want to print a portion of the page.
BarTab Lite
Keeps tabs from loading until you activate them.