For Gmail aficionados, one must-have Firefox add-on is the Better Gmail extension from Lifehacker editor Gina Trapani. It's basically a collection of her favorite Greasemonkey scripts, nicely packaged for all of us too lazy to install Greasemonkey and all of the individual scripts separately.
The list of what it does is long, so we'll try to summarize. It integrates Google Reader into the Gmail interface. It converts the attachment paper clip to an appropriate icon. It gives you a conversation preview window, a search-by-date option, a filter creation assistant, and label colors. It provides several skins (although we liked Google's original layout the best) and it lets you save search strings. It contains an attachment reminder.
It does not do your dishes.
You can easily turn on or off a feature through the preferences window, which brings us to one of the drawbacks of the add-on: it can only be accessed through Firefox's extension management window. More specific information and instructions are available at the Lifehacker site, and are essential for knowing how to use some of the features, like label colors. Despite these quibbles, we loved this extension and found it extremely helpful.