These are some useful firefox security extensions which helps you take control of your security.
The Facebook Container is very popular and useful extension that isolates facebook in its own session container and prevents unncecessary tracking. Once installed it then automatically loads facebook in its own container.
HTTPS Everywhere is another popular and useful extension that automatically enables https on supported sites, even if a user does not type it or follow a unsecure link thus, enabling encrypted connection.
Cookies Auto Delete is another popular security extension that automatically deletes unused cookies from the browser. Also, supports whitelising or greylisting with which you can also mark and keep cookies you trust. Supports container tabs extension.
Canvas Defender another powerful firefox extension that prevents canvas fingerprinting by using persistent noise technique which hides your real canvas fingerprint. You can find more about canvas fingerprinting at wikipedia Canvas Fingerprinting
Test canvas fingerprinting here at https://browserleaks.com/canvas
Privacy Badger automatically blocks invisible trackers by logging all domains that seems to be tracking the user. It also blocks link click tracking information from reaching tracking sites.
Avast Online Security is another popular security extension that protects users against known phishing and malware sites, helping improve browsing experience using crowd sourced web reputation and rating system.
Anonymox another most popular and most useful extension for firefox, that integrates a proxy in just one click thus anonymizing your activities online. With this extension you can change your ip-address and country, visit blocked or censored websites.