GTDInbox (formerly GTDGmail) transforms Gmail into a powerhouse of productivity and manageability. It gives you a faster, more task-orientated, better inbox.
The following from "Firefox Add-ons"...
FASTER DAILY USE OF GMAIL
Do your most common functions within one or two intuitive clicks using a new popup command box that lets you preview, archive, delete or modify labels on your emails.
TURN EMAILS INTO TASKS
Mark emails as actions to be completed and give them a priority. Process actions by project, context or person.
BROWSE GMAIL AS A PERSONAL INFORMATION DATABASE
Categorise actions, communication and resources (e.g. files, meeting plans, invoices) into meaningful clusters - such as by project, context or contact - for easier information discovery and retrieval later.
GIVE GMAIL NEW FEATURES
Preview messages, pre-label messages before you send them, multi-task by easily opening new windows, and more!