16 Must-Have FireFox Add-ons

Download Statusbar [home page]
It is a browser extension that allows you to keep track of ongoing and completed downloads in a hide-away status bar.

FireFTP [home page]
It is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers.

FlashGot [home page]
It enables you to integrate Firefox with download managers.

Foxmarks [home page]
If you use multiple PCs you will love this add-on. Foxmarks is a bookmarks synchronizer which allows you to synchronize your bookmarks across multiple PCs.

Forecastfox [home page]
It is an extension that brings international weather from AccuWeather.com to your Firefox web browser

Greasemonkey [home page]
It is a Firefox extension that allows you to customize the way webpages look and function. Hundreds of scripts are already available for free.

IETab [home page]
It is an extension that embeds Internet Explorer in a Mozilla/Firefox tab. There is no need to launch Internet Explorer separately! You can configure which websites should automatically open in an IE Tab.

iMacros [home page]
Just automate your clicks using iMacros. It takes care of repetitive tasks quite easily.

MeasureIt [home page]
A tool for website developers which allows you to draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.

PDF Download [home page]
With this extension, every time you click on a link, it checks if the target is a pdf file and in this case let you choose what you want to do (open pdf file inside or outside Firefox, download it to the file system or view it as HTML).

Screengrab [home page]
It is an extension for Firefox that makes it easy to save a web-page as an image. With it, you can save anything that you can see in a browser window - from a small selection, to a complete page. There is no need for expensive tools to do this when you have a free extension embedded right inside Firefox!

SearchStatus [home page]
It lets you view its Google PageRank, Google Category, Alexa popularity ranking, Compete.com ranking, Alexa incoming links, Alexa related links and backward links from Google, Yahoo! and MSN.

Split Browser [home page]
This splits the content area of the browser window as you like. It will help you in various cases. For example, to compare multiple webpages, to show a calendar always, and so on.

StumbleUpon [home page]
Do I really need to say much about what StumbleUpon is?? Get the add-on for Firefox.

Tab Mix Plus [home page]
Enhance your tabbed browsing with this add-on.

Web Developer [home page]
It adds a menu and a toolbar to the browser with various web developer tools. A must for all website developers.

I personally use all the above add-ons and many more!

UPDATE [20-Nov-07]: I have left out AdBlock Plus from the list, since I personally run a lot of websites with the aim of monetization using advertisements. I have mixed feelings of using an add-on like AdBlock due to that! Just a personal opinion though.