Useful Firefox Add-ons

Beyond privacy related addons, here's a list of a few that I highly recommend:

Dark Reader

This eye-care extension enables night mode creating dark themes for websites on the fly. Dark Reader inverts bright colors making them high contrast and easy to read at night.

DF Youtube (Distraction Free)

Allow yourself to focus while using YouTube for work, recreation or education.

Disable tab-detach

Returns a tab to its original window and position on detach.

Imagus

Enlarge thumbnails, and show images/videos from links with a mouse hover.

Iridium for YouTube

Allows the tuning of a huge amount of extra features for YouTube.com

JSON Lite

Fast JSON viewer - highlights, shows items count/size, handles large files.

LiveReload

Allows hot reloading of local development sites.

Redirector

Automatically redirect pages based on user-defined rules, supports RegEx.

Tabliss

A beautiful New Tab page with many customisable backgrounds and widgets that does not require any permissions.

Vim Vixen

Allows you to navigate with vim-like bindings.