Some serve same basic purpose, but only use one at a time. Uncertain which I prefer or haven't found better ones. Divided into Currently Installed, Uninstalled or Disabled, & Interesting Add-ons.
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
YouTube High Definition is a powerful tool that will let you automatically play all YouTube videos in HD (High Definition), turn off annotations, change video player size, auto stop videos, auto mute videos, control embedded videos and much more.
Shows full image when you hover over a thumbnail. Works with Amazon, Baidu Images, Bing Images, Facebook, Flickr, Google+, Google Images, IMDb, LinkedIn, Netflix, Pinterest, Reddit, Twitter, Yandex, YouTube, Wikipedia, Yahoo Images, & many more.
NoSquint allows you to adjust the text-only and full-page (both text and images) zoom levels as well as color settings both globally (for all sites) and per site.
LeechBlock is a simple productivity tool designed to block those time-wasting sites that can suck the life out of your working day. All you need to do is specify which sites to block and when to block them.
Allows to customize or disable most of the shortcuts offered by Firefox. This add-on does *not* allow to add new shortcuts, but it might help you discover existing ones.
Make any pages' search functionality available in the Search Bar (or "search box")...
Allows URI texts written in webpages to be loaded by double clicks.
Customizes the new tab page. Add more tiles. Add the launcher from Firefox start. Have a background image. Set new tile images and titles. See recently closed tabs. And more...
Expands the context menu's 'Search for' item into a list of installed search engines, allowing you to choose the engine you want to use for each search.