In response to that article I'll write my own Top 20 list. I'll also tag each extension as either "must-have" (MH) or "nice-to-have" (NtH). MH extensions are extensions that will/should benefit everyone and NtH extensions are those that really depend on each persons individual needs.
- Adblock Plus - MH
This extension is without doubt the most important of them all. It increases effectiveness because you don't have to be distracted all the time by annoying ads on every webpage. Sure, I've heard people say they don't even notice the ads when they're browsing in Internet Explorer, but give them a week without ads and they'll never want to go back.
- Adblock Filterset.G Updater - MH
This is an extension to the Adblock/Adblock Plus extensions. It uses a central filter list so you don't have to update and add filters for adblocking yourself all the time.
Edit: I was contacted by the author of Adblock Plus, who pointed me to this post that explains why Adblock Filterset.G Updater shouldn't be used. Thanks Wladimir :)
- Download Statusbar - MH
Firefox's built-in download handling just isn't that great in my opinion. That centralized download window that pops up just isn't all what it's crached up to be. Now this is definitely one of the extensions that's worth having, gives a nice overview of your downloads, and easy access to them as well while browsing.
- Image Zoom - MH
This is one of those that might be a Must Have on a personal level, but I still think it's very useful for everyone. It adds the ability to right click any image on the web page an either zoom in or out. Very useful if you're reading a forum where someone posted some super big image that just destroys the readability for all posts on that page.
- Tab Mix Plus - NtH
This one is definitly not a "must-have", this is a personal choice for me over ChromaTabs and Colorful Tabs. It adds some other session features than what is built-in to Firefox 2, and also adds some very nice Tab features, such as locking a Tab. Adding close tab buttons to all tabs, regardless of number and much more.
- IE Tab - MH
I guess this could be classified more as a "Nice-to-Have" than a "Must-Have", but I decided that this extension helps people be more efficient because they don't need to have 2 browsers open just to be able to browse a site that isn't developed properly.
- FaviconizeTab - NtH
This is one of those nice extensions that doesn't do much, but what it does helps a lot. I tend to have tons and tons of tabs open at once, this unfortunately lessens the overview of the tabs which can degrade efficiency. Some of all these tabs however, is easily recognizable by it's Favicon, so why not have the tab take up no more room than the Favicon?
- Google Toolbar for Firefox - NtH
I don't think this needs any introduction, I do however think some of it's features are overlooked. To begin with I didn't like this plugin because it took up room, it added an extra toolbar which I didn't really use. Now, just let Google Toolbar take over the Firefox built-in toolbar and you have one less toolbar to take up room. Then simply use Firefox's built-in Toolbar customization to add Google functionality anywhere on the other toolbars, especially the highlight/find search words is a near-must-have.
- DownThemAll - NtH
I rarely use this one, except if I'm going to download larger files. Being able to resume with this is gold, also being able to use multiple threads increases the speed quite a lot if the server supports it.
- RSS Ticker - NtH
If you like to keep up to date on news while browsing, this is a nice addition. Keep a nice overview on the latest while browsing your favorite sites.
- PDF Download - NtH
Don't you just hate when you click a link to a PDF document, and Adobe Reader opens up inside the browser, thereby removing your from the site you were reading? This will help you.
- MinimizeToTray - NtH
Sometimes I just have way too many windows open, and aren't really using the browser (Firefox), in these cases it's nice to make some room by minimizing the window to the Tray instead of the Taskbar.
- Tabs Menu - NtH
This is something I only installed because of Tab Mix Plus. The standard way Firefox 2 handles tabs, is that it makes the tab bar scrollable when too many tabs are open. It also adds a drop-down list of what tabs are open. Unfortunately Tab Mix Plus removes the drop-down feature, this is why this extensions becomes so handy, it adds a new menu entry with the list of open tabs.
- TinyUrl Creator - NtH
If you tend to paste a lot of long links, this will make it less of an eye-sore to those that have to see it, just right click the page you wish to get the link for and click "Create TinyUrl for this page".
- Favicon Picker 2 - NtH
This isn't that great really, more of a cosmetic addition. I like to have Favicons added to my bookmark entries since it gives a better overview, some sites unfortunately haven't figured out that there exists such a thing as Favicons. This is where Favicon Picker comes in.
- ColorZilla - NtH
Wonder what exact color is used in some part of the screen? Just click the eyedropper and select the color. Only really useful for Web Developers and Designers.
- Web Developer - NtH
Adds a ton of features that is very helpful to any person who creates websites.
- Firebug - NtH
Adds some of the same features as Web Developer, but the real benefit to this baby is that debugging websites just becamse so much easier.
- Auto Copy - NtH
This can be both a boon and a bane. If you're used to Linux Console style copying, or mIRC style copying, then this will certainly be useful. The downside is that sometimes it copies text that shouldn't have been copied, especially in some Rich Text editing environments such as TinyMCE.
- Adblock Plus: Element Hiding Helper - NtH
I haven't had a chance to use this, yet, but it sounded promising ;)