There are a lot of great add-ons (a/k/a: extensions). I use more than many people do, But, if I could only choose some of them, this collection represents them. This collection is a work in progress.
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
Collector's Note Once you start using this, you'll forget there were ads on the web. You will almost be shocked the next time you view the web without it.
The easy way to download and convert Web videos from hundreds of YouTube-like sites.
This works also for audio and picture galleries.
Collector's Note There are numerous add-ons that provide similar functionality, but for me, this is the best of the bunch. You can use it for quick and dirty downloads of video or Images from the web. I use it almost exclusively for downloading videos from YouTube. It's fast and efficient out of the box and you can tweak it beyond that if you like.
The first and only download manager/accelerator built inside Firefox!
Collector's Note Mass download tool. Download many files at once from a web page. You can also set it up to download one file in multiple chunks which generally speeds up the download. This is particularly handy for downloading very large files. In particular, I like it for downloading Linux distribution disk images.
Collector's Note Paradise for Power Users
Restyle the web with Stylish, a user styles manager. Stylish lets you easily install themes and skins for Google, Facebook, YouTube, Orkut, and many, many other sites. You can even customize Firefox and other programs themselves.
Collector's Note Moar Paradise for Power Users
Fight the text ads! Element Hiding Helper is a companion extension for Adblock Plus meant to make creating element hiding rules easier.
Never be annoyed by a Flash animation again! Blocks Flash so it won't get in your way, but if you want to see it, just click on...
Tab Mix Plus enhances Firefox's tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more. It also includes a full-featured session manager.
Remove or manage a new and uncommon kind of cookies, better known as LSO's.The BetterPrivacy safeguard offers various ways to handle Flash-cookies set by Google, YouTube, Ebay and others...
Latest updates: See bottom link 'version history'!
A customizable mouse gestures extension which enables to execute various commands and user scripts with six types of gestures.
When you find something on the web that you want to view later, put it in Pocket. It automatically syncs to your phone, tablet or computer so you can view it at any time, even without an internet connection.
IE Tab - an extension from Taiwan, features: Embedding Internet Explorer in tabs of Mozilla/Firefox...
Collector's Note I almost hate to include this in my "must haves" list, but when I'm using Firefox in Windows, this is among the first extensions I install. Unfortunately, a lot of web developers consider Internet Explorer the only worthwhile web browser. Therefore a (fortunately small) number of websites require you to use Internet Explorer for access (or to utilize all of the site's features). This add-on makes it easy by embedding IE in a Firefox tab. There is a "open page in IE Tab" option that appears on the context (right-click) menu making it quick and easy. And if you want to open the page in Internet Explorer, period, there is alos the "Open in Ext app" option on the context menu as well. This add-on also lets you tell Firefox to *always* use IE tab on a given site or page. All in all, it's very efficient.
Use PDF Download to do whatever you like with PDF files on the Web. Regain control and eliminate browser problems, view PDFs directly in Firefox as HTML, and use the all-new Web-to-PDF toolbar to save and share Web pages as high-quality PDF files.
Collector's Note I find viewing PDF files within Firefox annoying. I'd rather read them "offline" in a viewer (e.g. Adobe Reader). With this add-on, you just click on a link to a PDF file and up pops a box asking what you want to do with it (2 of the 4 options include download or view. Either way, it's default settings will cause the page to open up in your PDF reader instead of within Firefox).