Firefox has scored points with everyone who uses it for its implementation of add-ons called extensions. By opening itself for third party feature and enhancements integration, it has allowed users to participate in its development while keeping the core development simple and basic.
Extensions are probably single most reason for its popularity and fast user adoption.
Most popular and widely used extensions can be found on Firefox Add-ons site.
My must haves are: NoScript, AdblockPlus, Adblock Filterset.G Updater, SiteAdvisor, IEView, CustomizeGoogle, GoogleToolbar and TabMixPlus.
Dautovic 