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).
Adblock Plus Pop-up Addon extends the blocking functionality of Adblock Plus to those annoying pop-up windows that open on mouse clicks and other user actions.
Never be annoyed by a Flash animation again! Blocks Flash so it won't get in your way, but if you want to see it, just click on...
The most lightweight and easiest translator. Translate between more than 50 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.
Removes country restrictions for FREE! Just like that.
FEBE (Firefox Environment Backup Extension) allows you to quickly and easily backup your Firefox extensions. In fact, it goes beyond just backing up -- It will actually rebuild your extensions individually into installable .xpi files.
AddThis for Firefox is the best add-on to make sharing and bookmarking simple. Have all your favorite web 2.0 social networking, bookmarking, blogging, and e-mail services at your fingertips. Share any page, anytime, with anyone.
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 URI texts written in webpages to be loaded by double clicks.