Better Gmail
A comprehensive suite of Gmail enhancements, including skins, saved
searches, keyboard macros, secure access, attachment icons, attachment
reminders, label colors, signature floating and more.
https://addons.mozilla.org/en-US/firefox/addon/4866

Google Send to Phone
Google Send to Phone for Firefox is an extension that enables you to
send short text messages of web page content to your mobile phone. For
example, you might text message yourself a phone number, an address,
or directions that you find on the Web.
http://tools.google.com/firefox/sendtophone/index.html

All-in-One Sidebar
All-in-One Sidebar (AiOS) is an award-winning sidebar control,
inspired by Opera’s. It lets you quickly switch between sidebar
panels, view dialog windows such as downloads, extensions, and more in
the sidebar, or view source or websites in the sidebar.

https://addons.mozilla.org/en-US/firefox/addon/1027

GSpace
This extension allows you to use your Gmail Space (2.8 GB and growing)
for file storage. It acts as an online drive, so you can upload files
from your hard drive and access them from every Internet capable
system. The interface will make your Gmail account look like a FTP
host.
After the installation, you’ll get an option called “Gspace” in your
“Tools” menu, which opens the GSpace window.

http://www.getgspace.com/

del.icio.us Bookmarks
This extension seamlessly integrates your browser with del.icio.us
(http://del.icio.us/), the leading social bookmarking service on the
Web

https://addons.mozilla.org/en-US/firefox/addon/3615

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.

https://addons.mozilla.org/en-US/firefox/addon/1843

FireFTP
Along with transferring your files quickly and efficiently, FireFTP
also includes more advanced features such as: directory comparison,
syncing directories while navigating, SSL encryption, file hashing,
and much more!

http://fireftp.mozdev.org/

Web Developer
Adds a menu and a toolbar with various web developer tools.

http://chrispederick.com/work/web-developer/

Google Notebook
Clip and collect information as you surf the web

http://google.com/notebook

Backpack Publish
Highlight text in your browser, then click the Backpack Publish button
to post the selected text to a Backpack Note, Todo List Item, or
Reminder.

https://addons.mozilla.org/en-US/firefox/addon/3616

Backpack Pages
Displays a user’s Backpack (backpackit.com) pages in the toolbar.

https://addons.mozilla.org/en-US/firefox/addon/1544

Toodledo
Quickly add tasks to your Toodledo.com to-do list from anywhere. You
no longer need to signin to our website just to add a quick todo.

This extension gives you a contextual menu item that allows you to
select some text or right click a link to easily add it as a new task.

This task manager also includes a sidebar which keeps your most
important action items in clear view at all times.

An optional toolbar button gives you a one-click way to add a task.

https://addons.mozilla.org/en-US/firefox/addon/3789

GTDInbox
GTDInbox (formerly GTDGmail) discreetly integrates into Gmail making
it even more suitable as a GTD tool.

GTD - Getting Things Done - is a simple and effective productivity
concept: designed so that even the laziest and most scattered of
people can be organised and stress free.

https://addons.mozilla.org/en-US/firefox/addon/3209

Google Calendar Notifier
This extension provides robust notifications and display of your
Google Calendars for today, as well as showing any upcoming all day
events for tomorrow.

https://addons.mozilla.org/en-US/firefox/addon/2528

Delegate to Remember The Milk
Allows you to create RTM tasks from Gmail messages.