The name may sound a little dirty, but Greasemonkey is a superb Firefox extension that acts as a decorator and allows you to customize the look and feel as well as the behavior of any website you visit by means of custom Javascript files.
If you know Javascript, you can write your own scripts - or you can take advantage of the large number of user submitted scripts. Some scripts are generic and act on any website, while most scripts apply to specific websites.
Greasemonkey already solved my main Gmail annoyance: Gmail, in line with the idea of archiving all email and not bothering to delete any mail, does not offer a convenient Delete button or keyboard shortcut. The Gmail Smart-Delete Button Greasemonkey script addresses this very elegantly and offers both a context-sensitive Delete button and a corresponding keyboard shortcut. Very nice!