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 Flash Video Player is a powerful tool that will let you choose Flash video player as default YouTube video player.
The most lightweight and easiest translator. Translate between more than 80 languages with one click or a hot key. Translate from one word to a whole page.
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.
Controls as and when to display the Flash player and the HTML5 player.
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.
Make any pages' search functionality available in the Search Bar (or "search box")...
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 new ones.
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...