This list has been updated for FireFox 2 – Please see this updated list.

I’m a long-time user of the Mozilla and Firefox browsers. The base feature set is amazing but the extensions are what really make this browser a requirement. Here are my favorite extensions: Required (I use constantly):

• Tab Mix Plus: If I had to choose only one extension, this would be it. It makes the tabs behave any way you want them to behave. I use it to make child tabs open beside their parent, for the title text to be blue when the tab is unread, to show page download status in the tab title.
• Adblock + Adblock Filterset.G Updater: Great for removing unwanted ads in web pages; auto-updates the blocked sites list.
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• Download Status Bar: A great replacement for the annoying download window that pops up. Less obtrusive and it give the user a bunch of options to make downloads behave how ever you want them to behave.
• DownThemAll: A download manager built inside of Firefox – no external programs to worry about.
• FasterFox: Automatically modifies settings to speed-up Firefox.
• IE Tab: Can switch the current tab back and forth between IE and Firefox, all within Firefox. Great for sites that don’t support Firefox and for page design validation.
• OpenDownload: Not currently compatible with Firefox 2.0. Gives you the option to download a file to a temp location and launch it to the default Windows application. This is perfect for program install files that you don’t want to keep, so you don’t have to download install files to your desktop, run the installer, and then delete the file. Plus, it will automatically delete the temp downloads a few days later. This is similar to a feature offered by IE, one of the very few IE features I miss.
• QuickJava: Disable Java and Javascript quickly, in case you visit a suspicious site that you suspect may be running suspicious code.
  
• Update Notifier: The best way to see when there are updates to Firefox and any extensions.

Recommended for a Work Laptop:

• Webmail Compose: Integrates webmail in case you don’t have Thunderbird or any other mail client installed on that machine. Great for your work laptop.
• Proxy Button: Quickly turn the proxy on and off. Great for your work laptop.

Recommended (I use occasionally):

• Xinha Here!: Lets you launch a client-based WYSIWYG HTML editor. Uses the Xinha editor, but I would prefer TinyMCE or FCKeditor.
• Copy Plain Text: Will not copy formatting when copying text. Prevents the need to do an Edit / Paste Special / Plain Text command in the destination application.
• Google browser sync: great for bookmark backup, but there is so much more it can do beyond that.

Old & Obsolete or “Still On the Fence” (but I’m listing them anyway):

• StumbleUpon: Helps you find new sites that match your interests.
• AbstractMouse.com Shared Bookmarks: Lets you sync your between computers and access them online. Optionally allows sharing of bookmarks with friends or everyone via a webpage or an XLM feed. Wish it was better at synchronizing All Bookmarks instead of having to individually select each folder.
• del.icio.us: Not sure I get to popularity of this site, but it allows a user to mark, save, and share interesting web content.
• IE View or IE View Lite: Since switching to IE Tab, I don’t use these two. However, I do respect their small size and simplicity. These two programs let you open the current page in Internet Explorer (useful if case the web page author does not properly support Firefox).
• SpellBound (development version): Firefox 2.0 integrates a newer version of this extension, making this extension unnecessary. But if you aren’t using 1.5, it is an an awesome add-in for poor spellers. Allows built-in browser spell checking of any text entry field. Great if you post online frequently and it works much faster than pop-up spell checker integrated into some websites.
• Clone Window: This extension was made obsolete when this functionality was integrated into Tab Mix Plus. Open a new window or tab and it will use the history of the current tab, so the back button shows previously viewed pages from the first tab. Just like the behavior of the Internet Explorer “new window” function. The DublicateTab extension is an alternative; it requires right-clicking on the tab to be duplicated and clicking “duplicate tab”. It is a more manual process but may be better for those that do not want to automatically duplicate the existing tab every time a new tab opens.

One must-have utility for backing-up setting prior to a reformat or computer switch is MozBackup.