Firefox rocks because it acts like a Lego set -- you have your base module, and then you can stick extensions on it. Some of the most obvious extensions are toolbars. Unlike the embedded-application extensions like Chatzilla or FireFTP, toolbars actually modify your interface. A toolbar can contain search engine interfaces, bookmarks, RSS feeds, or extra tools for managing content. A lot of toolbars are very specialized -- tools for multimedia producers, musicians, and even Mormon theology students. Here are three more general toolbars that any net junkie will find useful.
The Google search engine has a number of query functions you can use to refine your search results, but most people don't know what those functions are. Even those of us who do know the syntax sometimes have trouble remembering what the right string is to get what we're looking for, and wind up paging through irrelevant results because it's quicker than looking up the functions.
The Google Advanced Operations toolbar exposes the search functions to the average user