Free and open source tool for syncing your browser data between browsers and devices whilst maintaining your privacy and anonymity.
Enhancer for YouTube is a very lightweight extension that is highly customisable, easy to use, and that will greatly improve your YouTube experience!
Auto Tab Discard a lightweight extension that uses the native method (tabs.discard) to unload or suspend browser tabs to significantly reduce the memory footprint of your browser when many tabs are opened.
A simple but powerful bulk/mass downloader. Download links from multiple tabs at once. Advanced filtering & multi-renaming options.
SingleFile is an extension that helps you to archive a complete page into a single HTML file or HTM-File with Google Chrome and Chromium.
Extension for Chrome and Firefox downloading both audio and video from many websites. Detects downloadable content and offers a user-friendly menu.
Download Them All! allows you to easily download twitter videos, facebook public videos and facebook private videos.No need to create an account, the only thing you need is the address (URL) of a Facebook or Twitter video. Download Them All!
Bypass Paywalls is a web browser extension to help bypass paywalls for selected sites.
Reddit Enhancement Suite is a community-driven unofficial browser extension for Reddit.
Send referers only when staying on the same domain. You can whitelist domains with wildcards and configure other things, look in the preferences page of the addon in the addon manager.
Easily open disposable containers that isolate attached data and are deleted after usage: Fully automatic, based on navigation-target, for certain websites, with configured mouse clicks on links or just by using the toolbar icon.
This extension helps you control more of your web activity from Facebook by isolating your identity into a separate container.
The Firefox Multi-Account Containers extension lets you carve out a separate box for each of your online lives – no more opening a different browser just to check your work email! Here is a quick video showing you how it works.
A simple web extension that redirects requests to privacy friendly alternatives. The successor fork is 
LibRedirect.
Protects you against tracking through "free", centralized, content delivery. It prevents a lot of requests from reaching networks like Google Hosted Libraries, and serves local files to keep sites from breaking. Complements regular content blockers.
HTTPS Everywhere is a browser extension produced as a collaboration between The Tor Project and the Electronic Frontier Foundation. It encrypts your communications with a number of major websites.
Open-source password manager with AES-256 encryption and device sync. Store and access encrypted logins across platforms like iOS and Android.
This efficient browser add-on blocks ads, trackers, and malware by default employing various filter lists, acting as a comprehensive wide-spectrum blocker.
