This is a thread to post add-ons you find useful for Mozilla Firefox and its derived browsers such as GNU IceCat. I'll keep my own list short and only include those which I find to be particularly helpful.
AdBlockPlushttps://adblockplus.org/Thought I'd start out with this classic. I'm assuming that if you use a Firefox-like browser you already know what this does, and if you don't then just read the name. It blocks ads.
Gnashhttp://getgnash.org/Gnash functions as both a standalone Flash player and a plugin for Firefox-like browsers. A great alternative to Adobe's proprietary plugin.
NoScripthttp://noscript.net/This is a neat little extension I just discovered. It's a whitelist Javascript blocker, meaning that unless you explicitly allow Javascript from a particular website to run, it will block it. It also blocks some other scripts, such as Flash Actionscript. Plus, you can specify certain websites as "untrusted" to place further restrictions on what they can do. A lifesaver for those concerned about the
Javascript trap, or just want privacy when browsing the web.
Screengrabhttps://addons.mozilla.org/en-US/firefox/addon/1146This quite simply turns a web page into a PNG or JPEG image.
VLC Media Playerhttps://addons.mozilla.org/en-US/firefox/addon/14730A pretty useful plugin which brings the functionality of VLC media player to your Firefox-like browser. Unfortunately, there is no way to make this plugin handle HTML5 embedded media.
WikiLookhttps://addons.mozilla.org/en-US/firefox/addon/7675Ctrl+right-click on any word to look it up on Wikipedia/Wiktionary. Sorry, Mac users.
All of the above add-ons are free software. For a longer list of free software add-ons for Firefox-like browsers, see
http://www.gnu.org/software/gnuzilla/addons.html.