The latest version of the Better Gmail 2 Firefox add-on boasts a useful new feature: a summary panel of unread email grouped by sender, as shown in the screenshot above.
In version 0.9 of the add-on, check off the "Summarize Unread Messages by Sender" checkbox (on the General tab) to see a listing like this one above your inbox, or any label or search result listing. Click on any name with the number of messages next to it to filter down to just those messages. This new user script is courtesy of developer Winston Teo, who explains how he uses this grouping to clear out his Gmail inbox.
Also new in version 0.9: a disabled-by-default "Support This Add-on via Amazon" option. When you opt into that feature, your purchases at Amazon kick back a small referral fee that helps support development on all the Better add-ons. Thanks in advance for your willingness to check that box. See the rest of the changelog or just download Better Gmail 2 version 0.9 here.