this is for myself to keep track for ff in multiple computers i use. it may also be beneficial for others to know.

web developer - a must have for any web developers
session saver - oh NO! my ff just crashed, now i'm going to lose all my open tabs. get this extension to recover all of your tabs when your reload your ff.
tab clicking - customize all your mouse clicking on the tabs how ever you like it.
scrapbook - sometimes i just want to save a portion of a webpage not the whole page. using this you can always just save the part of the page you selected and then organize your saved pages.
greasemonkey - for all of my hacking and customization needs ;) check out some of the scripts i wrote.
tabbrowser preferences - adds a tabbed browsing section to your tools -> options section that once you have, you can't live without.
forecastfox - get weather forecast while you surf.
downthemall - download all the selected images, files, media or links on page at once.
FLST (Focus Last Selected Tab) - brings focus to the last selected tab when closing the active tab. focus previous tab by clicking the active tab (tabflipping).
Ctrl+Tab Preview - This is an extension that replaces the behavior of Ctrl+Tab. Instead of directly changing tab it works some thing like Alt+Tab on windows.
adblock plus - block all types of ads.
dictionary search - highlight words & search
Download status bar - it replaces the default download popup
Firebug - if you do a lot of javascripting or html coding, this is a must have!
Dictionary Tooltip - (for FF 2.0) nice except it's charging now?
Foxmarks Bookmark Synchronizer - I love this plugin. It makes moving from computer to computer so much easier.
Video DownloadHelper - Awesome if you like saving streaming videos from sites like youtube.

Firefox tricks
Click links with middle mouse button to open them in new tabs.
If you don't have a middle mouse button, drap the link to the tabbar to open it in a new tab.
Double click on the tabbar to open a new blank tab.
Learn how to speed up firefox.
Firefox 2.0 tricks
How to config the close button on the tabs/tab bar


Other extensions i found that sound cool
colorful tabs - lively
colorzilla - gives r g b and hexcode of a color on a page. i don't like the fact you can't exactly copy and paste the values.
image zoom - zoom on an image without downloading it
gTranslate - With gTranslate you can translate any text in a webpage just by selecting and right-clicking over it.
bugmenot - BugMeNot allows you to access bugmenot.com's database of user-submitted usernames and passwords to bypass the login of web sites that require compulsory registration and/or the collection of personal/demographic information (such as the New York Times) via the context menu
IE tab - IE Tab - an extension from Taiwan, features: Embedding Internet Explorer in tabs of Mozilla/Firefox
Mouse Gestures - Allows you to execute common commands (like page forward/backward, close tab, new tab) by mouse gestures drawn over the current webpage, without reaching for the toolbar or the keyboard.

Other firefox related resources
Firefox support forum
Firefox shortcut keys
Firefox extension guru blog
How to write a firefox extension