Downloaders Anonymous: My 15 Essential Firefox 2.0 Add-ons
Adblock Plus: I don't like red-and-green flashing YOU ARE A WINNER!! ads any more than you.
BugMeNot: I don't like giving my e-mail address, much less taking the time to fill out yet another form and click on another e-mail confirmation link, any more than you.
DownThemAll!: DownThemAll! isn't just useful for downloading a ton of files at once (though that's most certainly its best feature)--it also makes a great general-purpose download manager for when Firefox's built-in options aren't enough.
ErrorZilla: Firefox's standard the-site-could-not-be-reached error message is so passé. ErrorZilla augments the lowly "Try Again" button with six more: Google Cache, Coral Cache, Internet Archive Wayback Machine, Ping, Trace Route, and Whois.
FoxyTunes: FoxyTunes 1.0 was great. FoxyTunes 2.0 is incredible. With support for controlling a zillion media players (even Pandora!) from within Firefox, skin support, lots of configurability, and a ton of thoughtful features like built-in lyrics search and fast player switching, this is one of the most solid add-ons available for Firefox.
Gmail Notifier: Google's official Gmail Notifier is plenty nice, and even a little more featureful than its Firefox namesake, but I prefer to get my notifications right inside Firefox. Productivity tip: Set Gmail Notifier's mail-checking interval to 30 minutes or more. If your e-mails are really more urgent than that, give the sender your phone number already.
Greasemonkey: If this list were in order of "essentialness," Greasemonkey would be in the top 3. With scripts available that alter the behavior and appearance of hundreds of sites, adding great new functionality and eliminating obnoxious stuff, it opens up a whole new web to you.
How'd I Get Here?: This is an innocuous little add-on that does one thing only: Tells you how you originally found the site you're looking at. It has an elephant's memory, so even if you bookmark the site and come back to it six weeks later, it will still be able to send you back to where you first discovered it.
IE Tab: Let's face it: Some sites are stupid. "Get with the program, people!" we can shout, but ultimately there will always be a few sites that are stuck in 1999 that just don't work right in our beloved browser. IE Tab is a lifesaver, allowing you to switch over to an Internet Explorer-rendered view of any page without leaving Firefox.
Live HTTP Headers: Another indispensible add-on for developers, Live HTTP Headers lets you see the traffic going in and out of Firefox, which can be a lifesaver when debugging wayward forms and Ajax actions, or just poking around other people's sites.
View Cookies: Firefox's Cookies manager does what it's supposed to, but View Cookies gives you a little more control. It adds a tab to the Page Info dialog that shows only the cookies for the site your looking at, and lets you do away with them at your whim.
Web Developer: I barely feel like the Web Developer add-on needs an introduction. Nowhere else does a more comprehensive suite of client-side tools for web developers exist. Whether you just write a little HTML here and there or if you code 10,000-line web apps, you need this one.
I had a hard time limiting this list. I may be addicted to Firefox add-ons, but that's what Downloaders Anonymous is all about, right? In case your appetite is as insatiable as mine, here's a few that I love dearly but didn't quite make the cut: del.icio.us, Download Manager Tweak, Nightly Tester Tools, SmoothWheel, and Stylish.
As always, we want to hear about your favorite downloads, so post your own Top 5 (or 15) in the comments!