Simple Mail

Website/Download: http://https://addons.mozilla.org/en-US/firefox/addon/5593
License: Freeware
OS: Windows Mac Linux
Latest Version: 2.74
Publisher's description:
Simple Mail is a mail client for Firefox. It allows you to get notified, read and write messages for multiple email accounts inside Firefox.

Configure one or more accounts in Preferences, set account refresh time and get notified of new mail. When new mail arrives, Simple Mail shows new messages count in status bar. You can click Simple Mail icon to open received messages in a separate browser tab. Also Simple Mail has a toolbar button, which you can put on toolbar to access SM easily.