Interesting add-ons that I don't use or have not installed
Lightbeam is a Firefox add-on that enables you to see the first and third party sites you interact with on the Web. Using interactive visualizations, Lightbeam shows you the relationships between these third parties and the sites you visit.
An indicator showing HTTP/2 and SPDY support in the address bar.
Adds the ability to create and edit entries in the password manager.
TinEye is a reverse image search engine. The plugin adds a context menu item that allows you to search for an image to find out where it came from, how it is being used, if modified versions of the image exist, or to find higher resolution versions.
Manages the Remote XUL whitelist to allow websites and local files to load XUL content.
Changes the Location Bar of Firefox into a Breadcrumb display with rich and interactive features.
Stratiform is the ultimate user friendly customization add-on for Firefox. It puts the power in the average user's hands to customize the browser's look.
An extension that helps with enabling and managing NTLM (Windows pass-through) and other SPNEGO (integrated authentication) settings.
Markdown document viewer.
Allows to visualize formatted markdown documents in browser.
Move Firefox Button to anywhere you want, and make it also available in Full Screen mode, or when Menu Bar is showing.
Suspends background old tabs automatically to save memory usage.
CSSsir is easy and handy tool which allows you to generate cross-browser code for main CSS- rules in a few clicks.
This CSS3 generator will be useful not only for web-masters and designers but also for common users.