Is there something you know, that we don't know? Have you been keeping it to yourself? Come now, (tusk-tusk)

Auto hide the Status bar – get an extra inch of screen space. You can customize the settings to have the status bar appear when hovering over a link or when a page is loading – including down to the milliseconds...
http://caspar.regis.free.fr/ahs/

The famous Google pagerank bar in the Status bar – no need for the toolbar to be open...
http://www.tapouillo.com/firefox_extension/

Resizable Form Fields – to simply drag and stretch a text area to write more comfortably...
https://addons.mozilla.org/en-US/firefox/addon/3694

Windows Shell Enhancer – Make those pull-down menus and task bar Translucent – this allows you to still see the browser page underneath the pull down menu or address bar, or form...(helpful when typing info from a page)
http://www.nuonsoft.net/shellenhancer/

> The famous Google pagerank bar in the Status bar – no
> need for the toolbar to be open...

Just a note, with a little customization you can also get the official Google Toolbar to show the PageRank in a pretty unobtrusive way...

http://blogoscoped.com/files/google-toolbar-pagerank-minimal.png

Nothing will ever beat Google Browser Sync on firefox extensions: it's the best program you will never see ;-)

Better Gmail is by far the best. (I also have a Gm. script to replace the Google Mail logo with the Gmail one).

Hey Martin Porcheron,

Do you want to share it with us?

Tiny Menu – Replace the standard menu bar with a tiny menu popup.
https://addons.mozilla.org/en-US/firefox/addon/1455

Pearl Crescent Page Saver – an extension for Mozilla Firefox that lets you capture images of web pages.
http://pearlcrescent.com/products/pagesaver/

<<Just a note, with a little customization you can also get the official Google Toolbar to show the PageRank in a pretty unobtrusive way...>>
http://img527.imageshack.us/img527/6088/21306846ye5.png
Choose "Customize..." and start drag and dropping.

http://img503.imageshack.us/img503/5302/67885371pz2.png
I packed all the stuff together lol. :)

Better Gmail – A few different enhancements for Gmail in one extension (also has some nice Gmail themes)
http://lifehacker.com/software/gmail/lifehacker-code-better-gmail-firefox-extension-251923.php

Google Mail -> Gmail logo – Much preferred to the Google Mail logo
http://userstyles.org/styles/872

Organize STATUS BAR – hide or rearrange icons that reside on the status bar ...
https://addons.mozilla.org/en-US/firefox/addon/1759

Page ScreenShot Capture – has more customizable features than most
http://pearlcrescent.com/products/pagesaver/

Or use this WEB BASED Tool – to take and archive an HTML version that is hosted
http://www.freezepage.com/

C'Mon Geeks, Contribute!!!

http://googlesystem.blogspot.com/2007/07/interclue-clever-link-previews.html

http://interclue.com/linkclues.html

Get CUSTOMIZABLE previews for any webpage with Firefox addon

I couldn't live without:
Firebug (great for those strange AJAX problems + CSS design)
Live Http Headers (recognize cloaking, strange redirects, etc)
Web Developper
Google Notebook
and of course my oyoy.eu search engine toolbar :-)