For bugs in Firefox Desktop, the Mozilla Foundation's web browser. For Firefox user interface issues in menus, developer tools, bookmarks, location bar, and preferences. Many Firefox bugs will either be filed here or in the Core product. (more info)
Currently, the call tree can't tell me how much time was spent idle. Instead, it shows me the top JS calls, but if we have enough idle time it doesn't even matter because they weren't expensive enough to make a difference.