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

Just head to Mozilla website at,
https://addons.mozilla.org/en-US/fir...rc=userprofile
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.

Features:
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