Browser Plugins

All these browser plugins work in Firefox, some also in Chrome/Chromium

Beginners

HTTPS Everywhere: Enables HTTPS by default on many websites,
straightforward and very easy to use (you basically forget about it)
Also available for Chrome, Firefox for Android & Opera.
More info about HTTPS Everywhere & Tor here

Ublock Origin: Blocks a huge amount of ads, same, you just forget
about it and never want to use a browser without it again.
It is available for Firefox, Chrome, Safari

Ghostery: Blocks a lot of cookies and trackers. May be too much if you
already use Privacy badger
Available for Chrome, Firefox, Safari, and Opera and as an app for iPhones, iPads, and iPods. It also works inside Firefox on Android phones.

Privacy Badger: Same as Ghostery, from eff so less intrusive. Maybe a
little more obscure to use. Also available for Chrome

 

Intermediate

Lightbeam: a Firefox add-on that uses interactive visualizations to show you the first and third party sites you interact with on the Web. As you browse, Lightbeam reveals the full depth of the Web today, including parts that are not transparent to the average user.

Calomel SSL Validation: Shows the quality of the certificate the
website you are browsing uses. Useless if you do not understand how a
certificate works.
Cookies Manager+: Very good to manage cookies, if you know how what
you want to do.

Advanced

Certificate Patrol: shows it when the certificate of a website
changes. Very noisy and needs a deep understanding of how certificates works
Certificate Watch: Shows more details on the certificate chain. Same
as cert patrol, you need some knowledge to understand what it is.
NoScript: Blocks javascripts by default, breaks most of the web2.0
websites, the user needs to know what they are doing.

 

Tor Browser Launcher

To install in an Ubuntu-based distribution
https://github.com/micahflee/torbrowser-launcher

 

Obfuscation

TrackMeNot: a lightweight browser extension that helps protect web searchers from surveillance and data-profiling by search engines

AdNauseam:  a browser extension designed to obfuscate browsing data and protect users from surveillance and tracking by advertising networks.