The Mozilla Toolkit is a set of APIs, built on top of Gecko, which provide advanced services to XUL applications. These services include Profile Management, Chrome Registration, Browsing History, Extension and Theme Management, Application Update Service, and Safe Mode. (More info)
The find toolbar is a pretty code browser feature, helping users to find content in the pages they view. There are a number of improvements Mike de Boer has identified that would significantly improve this feature.