Gmail Conversation Preview Bubbles - Firefox Grease Monkey Script
I have a problem with sorting my email - I set up filters, but never remember to archive everything. Plus, I'm plagued with a habit of marking an email as unread (when I'm busy) so I don't forget to reply - and then forgetting to check unread messages that have moved off-screen.
Fortunately, there's a great script for GreaseMonkey (a FireFox extension) that I've been using for a little while now. By giving the ability to right-click a message in your Inbox to preview, archive, or keep unread it really makes using Gmail much smoother and easier to use. The implementation is excellent and Gmail doesn't load any slower in Firefox for me than at all.
Gmail Conversation Preview Bubbles [persistent.info]
Resources Script (Right-click and Install User Script)
 GreaseMonkey Home
 Get Firefox
 Firefox Extensions from Mozilla.org