This section contains addons (themes and extensions) written for the Mozilla-related SeaMonkey and Firefox products. Addons come in 3 main varieties.
Extensions are major addons that add significant functionality to the browser. They usually go beyond just look-and-feel and actually change browser behaviour in some way.
Themes will significantly change the look-and-feel of the browser, usually moreso than the more lightweight (and also more recent) 'Persona' addon type.
Search providers are additions to your search box on the toolbar. They allow you to search on various different sites and search engines, for whatever the site/search engine allows you to search for - from books at Amazon to Google French language to Bugzilla bugs.