Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
See bug 1033577. That list is a global variable and is destroyed on exit. At that time, it's supposed to be empty, but it's not, i.e. some threads are not torn down before that point. "MemoryPressure" is one of them.