Every happy Firefox user prefers own set of extensions (aka add-ons or plug-ins). I have seen no one who doesn't use extensions at all. In this post I've collected a list that I find very useful. It can help you to discover an extension that you were unaware of and would regret missing it. All extensions are ordered by the subjective degree of their usefulness.

Usability

Tab Mix Plus

Enhances Firefox's tab browsing capabilities. I can't use Firefox without it. I hope all its features sooner or later will be added as options to the Firefox core.

Go to Selection

Adds the possibility to open selected plain text URLs in new tabs. It removes whitespaces from the selected text before opening it as an URL.

Go Up

Lets you go up a level to the parent directory on the current website by pressing Alt+UpArrow.

ViewSourceWith

Useful to view page source with external applications by pressing Control+Shift+U, edit textboxes content with your preferred editor and automatically see modified text on browser when you re-switch focus on it. As an editor I use GNU Emacs 23 setting the Editor Path to emacsclient and Parameters to -c $f.

FxIF

View EXIF data from digital cameras in image properties.

Searchbar Autosizer

Autosizes the searchbar as you type, cleans the query or reverts to your default search engine after submitting. Useful for saving space because I put Backward/Forward buttons, menu bar, URL box and search bar on the same line. You can see a screenshot here soon.

Context Search

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.

SmartSearch

Expands the context menu's Search for item into a list of keyword searches.

Image Mikirenaizer

The intelligent PageUp/Down, displays an image entirely. When you press PageDown while the lower side of the image is outside of the window, it will stop scroll down when the image top is in window top, and prevent upper side of image will be outside of the window.

Annoyance Removers

Flashblock

Blocks annoying Flash animations. When sometimes you want to see Flash, just click on a placeholder.

Adblock Plus

Even thought static ads and banners don't annoy me, sometimes I block some distracting ads on pages that I visit often.

NoScript

By default, it disables all scripts. I think this is too paranoid. I prefer "opt-out" instead of "opt-in" - I enable all scripts globally, and disable most annoying scripts, for example, that continuously scroll some parts of the page and thus eat CPU time, etc.

Nuke Anything Enhanced

Allows hiding of almost anything on the current page via context menu Remove Object or Remove Selection.

Inline blocked image view

Adds right click option to load blocked images inline, where should it be if image was not blocked. In other words, it reverts blocking for some selected images.

invertColors

Changes pages with dark backgrounds and bright text into the opposite. Currently it can't detect if a page has already a bright background to not change the html background color to black. So the only way to use it now is to disable it globally, and to enable a per-site override from the menu Tools -> InvertColors -> Always invert colors on this site to manually invert colors only on sites with black backgrounds.

keyconfig

Allows you to configure keyboard shortcuts. The problem with the default shortcuts it that the shortcut for closing a tab (Ctrl-W) is right next to the shortcut for closing the entire browser itself (Ctrl-Q). So hitting Ctrl-Q by mistake closes the browser and all of the tabs with it. With this extension, you can disable the Ctrl-Q shortcut.

Web Development

Web Developer

Adds a menu and a toolbar with various web developer tools.

Firebug

View, edit, debug, monitor CSS, HTML, JavaScript live in any web page. This is an irreplaceable tool for Web developers.

Firecookie

Cookie manager for Firebug.

FireScope

Extends Firebug with reference material for HTML and CSS.

FirePHP

Logs to your Firebug Console using a simple PHP method call instead of interspersing debug information throughout the content of your pages.

Live HTTP Headers

View HTTP headers of the current page.

Modify Headers

Add, modify and filter http request headers. You can modify the User-Agent string, add headers to spoof a mobile request.

XHTML Mobile Profile

Adds support for the mime-type application/vnd.wap.xhtml+xml that is not natively supported by Firefox.

wmlbrowser

Simulate WAP browsing by viewing WML (Wireless Markup Language) pages.

No-Referer

Open a link without sending the HTTP referer header. Useful for browsing server logs.

Greasemonkey

Customize the way a webpage displays using small bits of JavaScript.