Firefox add-ons provide better expandability over any web service. From a number of add-ons present, I have selected best 5 that would improve your experience with Google mail.

1. Gmail Manager

It is one of the most essential add-on that every Gmail user must have.Amongst the vast number of features that this add-on provides, some of the most important ones are:

– Manage and switch between multiple Gmail accounts.
– Check the number of unread mails without login.
– Receive mail notifications

2. Better Gmail

Another highly recommended add-on for Gmail. It is authored by popular blog Lifehacker editor, Gina Trapani. Basically it’s a collection of various Greasemonkey Scripts.

It adds useful features like:

– Hierarchy in labels
– File attachment icons
– Filter assistant
– Spam count
– Highlighting of messages on mouse hovering

3. Gmail Notifier

This is the original Gmail notifier meant for the Firefox. No need to install official Gmail notifier which pop-ups between movies. Now get your Gmail notifications within the Firefox window only.

4. WebMail Ad Blocker

If you feel that your Gmail inbox looks messed up, then this add-on is made for you.
This add-on removes advertisements from the right side pane of your gmail inbox. It even works for yahoo and hotmail.

5. Email This!

This add-on is handy in cases when you are on a webpage and would like to mail the contents. The mail recipients would receive mail with title as mail subject and url+highlighted text as mail body. Easy to use by shortcut keys and popup menu. Like it, mail it. As simple as that.

Bonus Add-on:

CookieSwap : A handy add-on to have multiple Gmail accounts working at the same time in single Firefox window. Since this add-on stores different cookies for Gmail at different locations, various accounts wont conflict with each other. Great, this could be used for other mail accounts like msn live and yahoo too