Gmail is perhaps the greatest web app ever developed, it is very popular (over 100 million users), has a generous 2GB of free storage (a small percentage of users will ever use more than this), and very, very user friendly. It is certainly powerful, but is it powerful enough? With Firefox Addons you can take your inbox even further…

GTDInbox Firefox Addon for Gmail

URL : GTDInbox
Description : GTDInbox is a powerful addon that will transform your Gmail inbox into an all in one manager that can compete with Outlook. You can do your most common functions within one or two intuitive clicks using a popup command box that allows you to preview, archive, delete or modify the labels of your emails. Another great feature is that you can mark emails as actions to be completed and given priority. Process actions by project, context or person.

Categorize actions, communication and resources…the features are seemingly endless, marvelous tool.

Better Gmail 2 Firefox Addon for Gmail

URL : Better Gmail 2
Description : Add useful extra features to Gmail, like hierarchical labels, macros, file attachment icons, and much, much more. Better Gmail 2 compiles the best Greasemonkey user scripts for Gmail into a single package.

tLo : Gmail Manager Firefox Addon for Gmail

URL : tLo : Gmail Manager
Description : This is a Firefox extension that allows you to manage multiple Gmail accounts and receive new mail notifications. Displays account details including unread messages, saved drafts, spam messages, labels with new mail, space used, and new mail snippets.

Gmail Skins Firefox Addon for Gmail

URL : Gmail Skins
Description : This cool allows you to customize the look of your Gmail inbox. Gmailskins adds the following features: Change the color/skin of your inbox, integrate your google homepage alongside your inbox (this allows you to view weather reports, rss feeds, news headlines, flickr recently added, google calendar module etc from your inbox), insert images from your Picasa web album in to your emails, insert smileys/emoticons, insert HTML tables and other complex HTML in to your emails, make the navigation (Inbox, Starred, Sent Mail, etc) appear horizontal, fix the navigation in place so that you don't have to scroll to the top of the page to see it, change the attachment paperclip (on inbox) to an icon indicating the type of attachment, and finally, hide various page elements (invite panel, page footer, your email address from the top right of inbox).
Indispensible extension.

Gspace Firefox Addon for Gmail

URL : Gspace
Description : Gspace turns the 2GB of your Gmail account into free online storage. With Gspace you can manage unlimited Gmail accounts to store all type of files within its simple, user friendly interface. Simply download Gspace and transfer files between your computer and Gspace, its that simple.

GCalQuickTab Firefox Addon for Gmail

URL : GCalQuickTab
Description : If you are using both Gmail and Google Calendar, GCalQuickTab makes it easy to switch between the two web apps by overriding the default behavior of the new Gmail/Google Calendar links that appear in the upper-left-hand corner of those sites. If the linked webapp is already open, then it brings that tab into focus instead of opening the webapp in a new window. You can also switch apps by using keyboard shortcuts: “g” for Gmail and “l” for Calendar.

GPhotospace Firefox Addon for Gmail

URL : GPhotospace
Description : This very useful extension makes use of your Gmail storage and uses it for online photo storage. Quickly and easily, any user can store their photos online for free. And best of all very little technical knowledge is needed.

Remember The Milk Firefox Addon for Gmail

URL : Remember The Milk
Description : Remember The Milk is a popular web app that allows you to manage tasks quickly and easily, with this Gmail addon you can add new tasks (and connect them with your emails, contacts, and Google Calendar events) and automatically add tasks for starred messages or specific labels.

Gmail S/MIME Firefox Addon for Gmail

URL : Gmail S/MIME
Description : With the Gmail S/MIME extension, you can send and receive signed and encrypted S/MIME messages in Gmail. It not only secures your e-mails, it even protects your attachments.

Gmail Contact Book Firefox Addon for Gmail

URL : Gmail Contact Book
Description : With this simple, but useful addon,you can easily select your contacts from the Contact Book by clicking the little book icon next to CC and BCC Boxs.

Just Smile! Firefox Addon for Gmail

URL : Just Smile!
Description : This extension replaces text-smileys with the image-smiley variation that that you want (the smiley list is editable).

TodoCue Firefox Addon for Gmail

URL : TodoCue
Description : TodoCue is a free web app for creating simple todo lists, simply download the extension and your to-do-list will appear within Gmails sidebar.

Gmail Notifier Firefox Addon for Gmail

URL : Gmail Notifier
Description : Another small addon that allows you to monitor multiple Gmail accounts right inside your browser's UI.

Easy Job Application Firefox Addon for Gmail

URL : Easy Job Application
Description : When searching online for a job, and you need to quickly and painlessly send a job application or CV, then this addon is for you. “Right Click” and select “Send Job Application to…”, and that is it, you have applied for a job.

WebMail Notifier Firefox Addon for Gmail

URL : WebMail Notifier
Description : WebMail Notifier checks your webmail accounts and notifies you of the number of unread emails.

Blank Canvas Firefox Addon for Gmail

URL : Blank Canvas
Description : This tiny extension automatically inserts HTML signatures into your Gmail messages based on which address you are sending from. Works for Compose Message and Reply/Forward.

Email This! Firefox Addon for Gmail

URL : Email This!
Description : Email This! will send your recipient the link, title, & highlighted text of the page you are viewing via GMail.

Send Page By Email Firefox Addon for Gmail

URL : Send Page By Email
Description : Send web links, send text, or send an image on any webpage you are currently browsing, and email it to anyone you want.

Todoist Firefox Addon for Gmail

URL : Todoist
Description : Todoist is a popular online task manager that is useful, fast and easy to use and it also comes with an addon that integrates easily into Gmail.

Gmail Checker Firefox Addon for Gmail

URL : Gmail Checker
Description : A simple tool that checks for new emails in your Google Mail and Google Apps (Gmail for Domain) accounts.

WiseStamp Email Signatures Firefox Addon for Gmail

URL : WiseStamp Email Signatures
Description : WiseStamp enables you to easily customize & add personalized email signatures on any webmail service (Gmail ,Yahoo Mail ,AOL mail, Hotmail, Google Apps…).

MyWebber Firefox Addon for Gmail

URL : MyWebber
Description : This extension gives you more screen space for your messages, Gmail advertisements take up a lot of space even if they get blocked by other addons because they leave unused empty space.