We have looked at the ten most popular search extensions for Firefox, however we can not overlook the many popular search engines you can add to the Firefox search box.
Most Popular Search Plugins for Firefox *

YouTube – Find all kinds of cool web videos.
IMDB – Quickly find that one actor that was in that movie you just saw.
Wikipedia – The web’s encyclopedia.
Album-Cover-Art.org – Quickly find album art from your favorite musical artists.
isoHunt – BT search – Find ISO images [...]

Searching in Firefox is a very easy thing to do.  Just type a phrase into the search box in your upper-right corner of the browser and hit the “Enter” key.  Now the trick is, how can we make searching in Firefox better?  Here are ten of the most popular Firefox add-ons to revolutionize the way you search the Web.

Googlepedia – delivering relevant Wikipedia articles along with your search results.
Google Assistant – Surf Canyon – helps to unleash the power of [...]

As the author puts it, capturing the theme of the 80’s era spirit, Hexxya! for Firefox looks to be a really unique theme that is very easy on the eyes. 
I really enjoy the simple icons and the softer use of colors in this theme for Firefox.  It all just works together very well.
Here is more about the theme from the developer:
You might notice that random arrangements of icons will not work with this theme. You will need to play [...]

Want to discover great web sites without the work of searching for them?  SimilarWeb, an add-on for Firefox, will do the searching for you.  Its job is to suggest similar web sites you might enjoy as you browse around the Web.
I am always looking for new ways to find new and interesting sites out there so this is an add-on I really enjoy.  There are several ways to customize the way you view the similar sites too, that are [...]

Ready to make your Firefox browser a little more Wii-like?  WiiFox, a new theme for Firefox, is based on the highly popular video game system the Nintendo Wii.  It doesn’t really rip anything off directly, it just takes inspiration really from the Wii’s own internet browser and interface. 
If you like the theme, be sure to let the developer know.  Since it is right now an “experimental” theme, I am sure feedback would be appreciated in any way you can [...]

Thanks to Firefox and some of the great add-ons that have been developed for it, your browser is your best friend when it comes to web site design.  From CSS tools, to promoting your domain, here are 10 of the most popular web designer tools for Firefox.
Any of these add-ons would be great to add to your own proverbial web designer’s toolbelt.

ColorZilla – This advanced eyedropper and color picker will help you find out exactly what color is being used [...]

Have you ever looked at a web page and thought to yourself, “it would look better if I could only move this over there, or change the color of that or..” well, there are a lot of things you could change about a lot of the Web’s most popular web sites.  One tool for Firefox lets you customize the look and feel of Firefox and the Web, Stylish.
Using what is called “user styles” the Stylish add-on for Firefox can [...]

It has been a long time since I have taken a look at Tab Mix Plus.  I know a lot of people who use it, and it continues to be one of the most popular Firefox add-ons out there.  So the first question to ask is, what makes it so darn special?
Well, the main goal of Tab Mix Plus is to give Firefox’s tabs more capabilities.  You can quickly duplicate tabs, control the tab focus, you get all kinds [...]

Since the recent Facebook re-design, there are not many themes or skins out there that work with it.  However, I have found one that is fully compatible and (for some) a much more pleasant viewing experience, the Vista Ultimate Skin for Facebook.

Vista Ultimate Skin for Facebook – Want to make Facebook look a ‘bit more Vista-like?  This user style will give Facebook the look and feel of Windows Vista.
Also, if you want just the Vista-like taskbar inside of Facebook, you [...]

If you need to edit some of the advanced preferences of Firefox, and you need a neater way to get that done, gui:config is right for you. 
This extension of Firefox will give you a lot of the “about:config” preferences, in an easy to navigate menu box.  It displays the options, included in the current version, using checkboxes, dropdown lists and textboxes.
If you want to check out all the advanced settings that gui:config makes available to you, Lifehacker has [...]