There are hundreds of useful (and most of the time not) Firefox extensions to improve your browsing experience as well as bog down the browser’s performance. I choose 4 out of the flock and limit myself to them, to balance work-ability as well as the bulkiness of my Firefox install.

All-In-One Sidebar
Available via: he AiOS Repository or Developer site

Why?
If you’ve tried the Opera browser, you’d love the sidebar that’s well integrated into it. It holds everything you need like bookmarks, downloads, history, plugins/widgets, etc and you can hide the darn thing at the click of a button. All in One Sidebar (or AiOS if that seems a bit long) brings that functionality to Firefox.

If you still don’t get the beauty of this plugin in, try this scenario: You’re constantly checking your downloads, browsing your favorite site and looking for bookmarks that relate to what you are working on, all at the same time. You may get away with checking your bookmarks but I’m sure you’ll opt for a simpler way of checking your downloads without Alt-tabbing your time away.
the edge again hides AiOS again.

How?
AiOS opens your Bookmarks, History, Downloads, Extensions, Themes (and some other some other windows which not many people use) within itself, a pane you can place on the left or right side of your browser. Each side bar panel is also available by a shortcut key so that you can quickly pop out the side bar with just one keyboard combination.

The Extensions panel is probably one of the most useful panels with AiOS. You can see which extension has an update available, and configure the extension by right-clicking on it and selecting the appropriate menu item. You can disable / enable any extension from the popup menu too, but this still requires a restart of Firefox.

Bookmarks couldn’t have been more conveniently placed. You can quickly scroll through your bookmarks and select which one you want to open. The sidebar stays open after you click on a bookmark link, so you can open your favorite bookmarks one by one.

DownThemAll!
Available via: DTA Repository or Developer site

Why?
We all love to download stuff. Pictures, mp3’s, videos etc, custom content for your Sims 2 installation. Isn’t it a pain when you have to download 50 or more of them from a single page?

DownThemAll (or DTA) does away with that and gives you a way to download multiple items in a breeze.

How?
DTA’s selection screen allows you to choose by extension (say, if you’re downloading pics, lots of pics) and download them to a specified folder. You can use the Renaming Mask if you want to rename the files immediately after download.

Using Filters allow you to customize specific actions for any file type as well as pick out specific files from a long list of downloads with regular expressions. You’ll find much use for DTA when browsing imageboards.

Adblock Plus
Available via: Adblock Repository

Why?
Many users switch to Firefox specifically for the Adblock. Why? For every eye-catching ad we see on the net, there’s always one that either insults your intelligence or let people behind you think that you’re browsing porn. Then there’s the usual pop-ups that most of us love so much to hate.

Adblock Plus gives you the option to block every ad in sight with a press of a button (the Adblock plus button is located somewhere in your navigation toolbar, depending on your theme.)

How?
Compared to the original Adblock, Adblock Plus is a bulkier build because it conforms to a much stricter set of options. Adblock works via the Content Policies that Gecko, the engine on top of which Firefox, Thunderbird and other applications are built. It configures one of these built-in content policies and the application logic takes over, deciding which objects, be it javascript or just an image, to block and user interface code to allow configuration of filters.

The filters are update-able. You can choose which filter to add your Adblock Plus plugin.

Adblock Plus offers exceptions, allowing you to register sites which you don’t want to be blocked.

TabmixPlus
Available via: Tab Mix Plus Repository

Why?
Tabbed browsing is one of the primary reasons why Firefox is so popular. Even if you’re just getting started with Firefox, as soon as you do open a bunch of Tabs, you will wish that there was one particular behavior of the Tabs that you could customize. You may want new tabs to open into the foreground and enter focus and next to the current tab (just like IE7). You may want to size them so that they are smaller/bigger. Perhaps you don’t want the close Tab button, because you only use the Ctrl + W or the Ctrl + F4 shortcut keys and would love the extra space on the Tabs bar.

Maybe you even want the progress of the page load to be displayed on the tab, somehow, instead of the standard rotating Firefox dotted circle.

Tab Mix Plus lets you do all these things, and much, much, more.

How?
Session Manger is a nightmare to configure, and the sheer amount of options available to Tab Mix Plus might turn you away. But rest assured, most changes will only impact the way your Tabs look (and open, close, etc.) and you can always go back and configure it differently. Thankfully Tab Mix Plus will let you restore it to the default settings and even let you import / export them so you have a sort of Save Point to start over when you screw something up.

I recommend setting the Tab opening order to Open new tabs next to the current one, and change opening order as well. This will cause any new Tab you open, to open beside the current one; changing the opening order will enhance this further so that the order of Tabs opened is retained – the First Tab you open will open next to the current, second will open next to the first instead of the current, again.

When you close a Tab, which Tab becomes current next is governed by the Closing Tab Events in the Tab Mix options. Set this to Left or Right according to your tastes and browsing habits.