Using too many plugins in Firefox will make an already resource hungry Firefox to get hungrier.
I make sure that I keep the add-ons down to a minimum. However, there are a few plugins I just need to have installed in my browser!
Adblock Plus
Ever been annoyed by all those ads and banners on the internet that often take longer to download than everything else on the page? Install Adblock Plus now and get rid of them.
FlashGot
Download all the links, movies and audio clips of a page at the maximum speed with a single click, using the most popular, lightweight and reliable external download managers.
Download Statusbar
View and manage downloads from a tidy statusbar – without the download window getting in the way of your web browsing.
Fast Dial
Fast Dial replaces blank tabs with a panel of thumbnails of your favorite sites.
FEBE
FEBE (Firefox Environment Backup Extension) allows you to quickly and easily backup your Firefox extensions. In fact, it goes beyond just backing up — It will actually rebuild your extensions individually into installable .xpi files. Now you can easily synchronize your office and home browsers.
FireGestures
A customizable mouse gestures extension which enables you to execute various commands and user scripts with five types of gestures.
PlainOldFavorites
Use Internet Explorer Favorites directly from Firefox. No need to import, export or synchronize – the same Favorites appear in Firefox as a new menu bar item.
FireBug
Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
SearchStatus
Display the Google PageRank, Alexa rank and Compete ranking anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, Alexa info and more.
Web Developer
Adds a menu and a toolbar with various web developer tools.
What Firefox Addons do you have installed?
This is a great round up. I would also add ForecastFox to the list. I love getting the weather report at the bottom of my browser.