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)
Opening and Closing the Find toolbar causes it to appear and transition below the currently visible toolbars. The animation has it fade in over the location bar or bookmarks toolbar if visible. It should fade in "behind" the top-level chrome.