Here are a few non-essentials that I personally love... Try them out if you think they would be useful to you, too.
Save time by speeding up repetitive tasks. Get faster, parallelized downloads, see definitions, auto-load the next page, improve searching, and more. FastestFox will dramatically enhance your productivity!
Personas are free, easy-to-install "skins" for Firefox. Personas Plus extends that built-in functionality to give you even more control and easier access to new, popular, and your own favorite Personas.
The most beautiful yet the simplest add-on that makes a strong colorful appeal. Colors every tab in a different color and makes them easy to distinguish while beautifying the overall appearance of the interface. For Seamonkey & Flock. An essential. F...
DuckDuckGo is a search engine that does not track you. Adds DuckDuckGo as the default address bar, search bar and right-click search engine. Adds a handy toolbar button for easy access to !bang tags. Adds DuckDuckGo instant answers to Google/Bing.
Multiple account (or label) Gmail notifier (without storing passwords)
Allows editing of web page content while in Print Preview mode, prior to printing or saving as HTML or PDF. Compacts the layout and removes unwanted content such as adverts, sidebars and blank pages. Any element can be formatted, hidden or deleted.
Make any pages' search functionality available in the Search Bar (or "search box")...
“I have read and agree to the Terms” is the biggest lie on the web. We aim to fix that. Get informed instantly about websites' terms & privacy policies, with ratings and summaries from the www.tosdr.org initiative.
Move tabs into the titlebar in a normal (restored) window.