Web-developer toolbar
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The web developer toolbar for Firefox is bar far the most useful Firefox extension that I have come across. I use it countless times every day, in the office and at home. It provides a wealth of powerful features that every “power” user and web developer will find instantly usable. The CSS editor is incredibly useful, especially if you are not very familiar with CSS, as it allows the user to see the changes reflected in real time in the browser. The whole package is worthwhile simply because of the CSS editor.
FireBug
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The FireBug website states “Firebug integrates with Firefox to put a wealth of web development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.” Personally I find FireBug invaluable for finding all the subtle nuances between the browsers. FireBug allows the user to “inspect” the properties of every single entity on a page, which is perfect when trying to figure out why IE insists on moving an object 40px to the left, no matter what you try to do.
FireFTP
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While a plethora of FTP clients available for free, I find FireFTP very easy to use. Furthermore, having a FTP client that integrates into the browser helps keep my desktop uncluttered while I’m working. It is very easy to install onto any computer that already has Firefox, providing an instant and easy to use FTP client anytime you need it.
IE Tabs
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This plug-in does exactly as its name suggests. Basically, it embeds IE inside a tab of Firefox, allowing all the advantages of IE(ahem if there is any;-p) within the confines of Firefox. This is particularly useful to web developers, or anyone trying to reach the Microsoft windows update page(not sure if M$ changed this, but for years you could only download windows updates with IE and thus SUN files an anti-rust suite along with most of Europe).
Delicious Bookmarks
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This may seem like one of the least useful plug-ins on the list, but I find that it very useful. Mainly, I use delicious bookmarks to keep a online version of my favorite sites, allowing for that list to be accessed from any internet capable computer. But Delicious can be taken a step further, I share my bookmarks with the people I work with. If I find an interesting article concerning design, I can save it and later, the design team can mull over it at their convenience.