I know that this has been written about on this site before; it is always good to go over it again. Here is a list of the add-ons that I find are the most useful in my everyday browsing in Firefox.
ColorfulTabs
This add-on is one of my favorites. It will color every tab in your Firefox browser in a different color and makes them easy to distinguish while beautifying the overall appearance of the browser. You can customize how and what color it will color each of your tabs.
Get ColorfulTabs
Firebug:
If you are a serious web developer or even just dabble with coding web pages, this is a must-have add-on. Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.  This means you can easily view and study other people’s code as well as your own.
Get Firebug
Flagfox:
This is a nice little add-on that displays a country flag depicting the location of the current website’s server and provides quick access to detailed location and web server information. This allows you with a quick look at your tool bar to see what country the web site you are viewing is hosted in.
Get Flagfox
Glazoom:
There are plenty of zoom add-ons for Firefox out there, but this is the one I use. I like the fact that there are numerous ways of zooming with this add-on. You can choose to zoom in to see a specific element from the context menu, hold down the ctrl key and scroll in or out using the roll wheel on your mouse, or use the dropdown menu located in your status bar.
Get Glazoom
Google Icon:
This nice little add-on is a great tool to enhance your Google searches. It places the favicon for each site that is returned in a Google search next to the site name.
Get Google Icon
Google Toolbar:
This is without question my favorite add-on. The toolbar allows you to do Google searches right from your Firefox toolbar. It also comes with lots of helpful tools like a spell checker, translator, page ranker, and Google Docs.
Get Google Toolbar:
History Submenus:
This wonderful little add-on places each day’s entire browsing history in a submenu of the history menu. How Firefox handles history was one of the things that I found the hardest to get used to when I switched over to Firefox from IE. This add-on does not completely mimic IE’s browsing history layout, but it is closer then the default Firefox method.
Get History Submenus:
Navigational Sounds:
An add-on that is a simple extension that plays the Windows OS start and end navigational sounds when pages start and finish loading (i.e. that “click” from IE when you click a link). Additionally, popup/blocked and information bar sounds are played. This is a nice add-on for people switching from IE to Firefox, or for those of us that like the IE sounds.
Get Navigational Sounds
New Tab Homepage:
This handy add-on opens your home page when you open a new tab instead of the default Firefox blank page or new tab page.
Get New Tab Homepage
Nightly Tester Tools:
This add-on is more for the advanced user. It allows users who are testing new builds of Firefox to override the compatibly restriction that a lot of add-ons have with beta or new builds of Firefox. This can cause problem with Firefox, so please use this add-on with caution 
Get Nightly Tester Tools
Personas:
This add-on allows you to easily change the theme for your Firefox browser. Personas are free, easy-to-install “skins” for Firefox. Choose from over 30,000 cool designs, including art from Harry Potter, Bob Marley and Lady Gaga to individualize your browser. You can even make custom themes for your browser.
Get Personas
Splash!:
This add-on adds a splash screen to most Mozilla and Mozilla-based products. Supports sound, opacity, CSS customization, custom images, and much more. You can add {appVersion}, {buildID} and {userAgent} to the loading text, and it will be filled in for you.
Get Splash!
Ubiquity:
This is a very powerful add-on that gives you a powerful new way to interact with the Web. The tool can do so much that it would be easier for me to just give you a link to a tutorial for it then try to explain what it does. This is one add-on you will not want to miss out on!
Get Ubiquity
Nice .what about Easy YouTube Video Downloader
I use YousableTubeFix a Greasemonkey script.