The official version 2.10 and the usual 2.11 will be detected as "not compatible with FF6". The file provided by Miguel below (a several posts down) will not be inactivated by the FF6 installer (because its is set to 6.0), but it does not work properly on FF6. When it is activated, I see only four recent histories and no submenus as used to be with FF5. I tweaked the settings a little but no avail.
Mozilla, please include the History Submenus function to the official FF.