This is a very simple yet efficient way for Firefox users,

Just head to Mozilla website at,
Install the extension.
There is not password storage in this extension for security concerns; instead it employs your current logged-in info. So to sign into multiple accounts at once, you need to first, sign in to a single Gmail account. Then, click your name or email address at the top of the page. Select Add account from the drop-down menu. On the page that opens, enter the email address and password for another account you wish to access, and click Sign in.

1. Multiple account support
2. Multiple label support
3. Low bandwidth usage by using RSS technology
4. Mark as read, report span, trash or archive massages right from Firefox toolbar