Marianne Lenox's "My Favorite Browser Add-Ons" post caught my eye, and I thought I'd post my own list. I'm intrigued by the fact that there was zero overlap between our lists.
When I get hold of a new computer for the first time, there are a few browser add-ons I have to install before I feel at home. Oh, and Firefox comes first. Do I even have to say that?
Adblock Plus: I'm always vaguely surprised when people complain about banner ads on the web, because I almost never see them. Adblock Plus does a brilliant job of keeping them off my screen.
Delicious Bookmarks: Replaces the old-fashioned on-disk bookmark list with easy access to my Delicious account. (I do use local bookmarks only on my browser toolbar, but my big list of bookmarks is all online at Delicious.)
Mouse Gestures: Allows you to execute common commands by using the mouse to draw simple "gestures" on the screen. I'm so used to using it for forward/back navigation that I'm often slightly confused when I try to navigate on a computer without it. This add-on has many more functions than I'm ever likely to learn -- I just find it handy for for forward and back browsing, and opening and closing new tabs.
LibX: I think of this as a counterpart to Zotero in many ways. LibX is an open-source library toolbar that can (must, actually) be customized for your particular library. It functions as a catalog/database search bar, but to me the coolest part is what it does to page contents. It adds library links to Amazon item records, auto-links ISBNs to your library's OpenURL server... Actually, it's worth a separate post in itself. One of my summer projects is to improve my Libx guide for GSU users.