Browser Extension and Add-On

Enhanced GitHub : Display repo size, size of each file, download link and option to copy file contents
Gitako - GitHub file tree : File tree for GitHub, and more than that.

Add custom search engine : Add a custom search engine to the list of available search engines in the search bar.
Selection Context Search : Right click on a selected text and choose the search website from the popup window or the context menu.
- Swift Selection Search : Swiftly access your search engines in a popup panel when you select text in a webpage.

Auto Tab Discard : Increase browser speed and reduce memory load and when you have numerous open tabs.
Containerise : Firefox extension to automatically open websites in a container
Containers theme : Change theme color based on your container color
Facebook Container : Facebook Container isolates your Facebook activity from the rest of your web activity in order to prevent Facebook from tracking you outside the Facebook website via third party cookies.
Firefox Multi-Account Containers : Multi-Account Containers helps you keep all the parts of your online life contained in different tabs.
Sidebery : Add-on for managing tabs, containers (contextual identities) and bookmarks in sidebar.

Bitwarden - Free Password Manager : A secure and free password manager for all of your devices.

Javascript Switcher : Add a button in the url bar to block or enable javascript per-site.
ClearURLs : Remove tracking elements from URLs.
HTTPS Everywhere : Encrypt the Web! Automatically use HTTPS security on many sites.
uBlock Origin : Finally, an efficient blocker. Easy on CPU and memory.
Google Direct : Remove tracking links from Google Search results

Refined Hacker News : Add useful features and tweak a little stuff to make the HN experience better without changing the look and feel
Dark Reader : Dark mode for every website. Take care of your eyes, use dark theme for night and daily browsing.
Wikiwand: Wikipedia Modernized : Good old Wikipedia gets a great new look
Violentmonkey : An open source user script manager that supports a lot of browsers

Epiverse : View Reddit and Hacker News comments on all webpages.
- Thredd - Useful Advice from Reddit : Get reviews for the page you're reading or recommendations for similar content - crowdsourced from Reddit!
Reddit Enhancement Suite : A suite of modules that enhance your Reddit browsing experience

Find Code for Research Papers - CatalyzeX : Find code implementations for machine learning research papers with code directly on Google, Arxiv, Scholar, Twitter, Github, etc.

What Hacker News Says : Easily find Hacker News threads about the page you're currently browsing.

Copy Selection as Markdown : Copy title, URL, and selection as Markdown
markdown-clipper : This extension works like a web clipper, but it downloads articles in a Markdown format.
Open in Sumatra PDF and DJVU Reader : Adds a context menu item to send PDF links directly to Sumatra PDF
Open in VLC™ media player : Adds a context menu to send audio and video streams directly to the well-known VLC™ media player
SingleFile : Save a complete page into a single HTML file

Mendeley Web Importer : Fast, convenient import of references and PDFs to your Mendeley Reference Manager library
NoteBook Buddy : Provides a helping hand when using Jupyter NoteBooks

Mind the Time : Keep track of how much time you spend on the web, and where you spend it.
Pomodoro clock : A simple pomodoro clock in your browser to hack your productivity.