Most of us own more than one gmail account , one of the most annoying aspects of dealing with it is the fact that you cannot have more than one opened at the same time. Luckily, Firefox has a plugin, called Gmail manager , it is an add-on for firefox that allows you to manage multiple Gmail accounts
Just download plugin from this site and install.

2) Mail Fetcher

Mail Fetcher is very useful google feature , which can heck up to five other email accounts and download all that mail into your Gmail. Setting up Mail Fetcher is easy, just follow these steps:

-Click Settings at the top of any Gmail page, and open the Accounts tab.Click on the “Add another mail account link.”

-Enter the email address of the account to get mail from and click on the Next Step button.

-Set details for your new email account such as your password, POP server, port and label incoming messages

After this you’ll be able to retrieve mail from this account.

3) Bookmark any label, folder or message type

In this simple tip will see how to create one-click access to all your Gmail labels, Inbox, Sent Mail, Drafts…
Go to Labs category and enable Quick Links. Click on Save Changes

Select desired label, search or message and click Add Quick Link in the Quick Links box.

Type a recognizable name for the bookmark and click OK

4) Quick gmail hacks
-press “m” key to mute a conversation
-label:unread label:inbox show only unread message
-drop attachments directly into the attachment box with dragdropupload 1.6.8, firefox extension
-Ctrl+s save the current email as a draft
-Speed up gmail , just press F8 (turn off message pane)