This is pretty cool. Say you have an often-performed search on your GMail archive – you can store that as a Persistent Search via a Greasemonkey plugin to Firefox.
Persistent searches (a.k.a. smart folders or saved searches) seem to be the feature du jour of email clients. Thunderbird has them, Evolution has them, and Mail.app soon will. On the other hand, Gmail is the web mail app to use. While one doesn’t normally think of web apps as having such advanced power user features, it recently occurred to me that it should be possible to add persistent searches to Gmail.