Throughout my use of Firefox, I've aquired several extensions that I feel I should share with the rest of you. I request that this topic be sticked if others decide to post their extensions, so that we have one general place to look for them, and not many different topics.
Flashgot
Download one link, selected links or all the links of a page at the maximum speed with a single click, using the most popular external download managers for Windows, Mac OS X, Linux and FreeBSD (dozens currently supported products, see http://www.flashgot.net for details). FlashGot offers also a Build Gallery functionality which helps to synthetize full media galleries in one page, from serial contents previously scattered on several pages, for easy and fast "download all".
Adblock
Once installed, it's a snap to filter elements at their source-address. Just right-click: Adblock: done. Filters use either the wildcard character (*) or full Regular Expression syntax. Hit the status-element and see what has or hasn't been blocked. (In other words, if you see an image or flash animation you don't like, right-click it and permanently remove it.)
Internet Explorer view
Adds "View page in Internet Explorer" links to the content and link context menu. Handy for previewing pages in IE, loading up IE-only pages when you run across them in Mozilla, etc.
Page update checker
(I don't think I need to copy and paste the explanation for this one, if you are on these forums, you know why this is so useful.)
Dr. Web anti-virus link checker
This plugin allows you to check any file you are about to download, any page you are about to visit with online version of Dr.Web anti-virus! New menu item "Scan with Dr.Web" will appear on hyperlink context menu.(This is a plugin, not an extension. But it's useful enough for me to post it here.)