The config.trim_on_minimize preference no longer exists, as it was Windows XP's ...

The config.trim_on_minimize preference no longer exists, as it was Windows XP's own behavior that Firefox was suppressing:

https://bugzilla.mozilla.org/show_bug.cgi?id=660367

  // Our internal trim prevention logic is effective on 2K/XP at maintaining
  // the working set when windows are minimized, but on Vista and up it has
  // little to no effect. Since this feature has been the source of numerous
  // bugs over the years, disable it (sTrimOnMinimize=1) on Vista and up.