Stefan,
With your Snow Leopard operating system you'd need to use ClipWrap or Voltaic to convert a single .mts to .mov.
With recent Mac operating systems the accompanying QuickTime Player can open the .mts file and instantly save it as .mov. With QuickTime Player 10.2 that comes with OSX Mountain Lion, you'd choose "export" and get an H.264 .mov file compatible with your FCP. With QuickTime Player 10.4 that comes with OSX Sierra, you'd choose "save" and get an acv1 .mov file.
An acv1 .mov file requires a workaround back in Snow Leopard. Old QuickTime Player 7 can open it; choose to save as reference movie. This reference .mov file can then be opened by old Compressor and there make a .mov file in whatever codec you wish to edit.
You can dedicate a small volume to a recent Mac operating system for such conveniences, while continuing to run old FCP on the older operating system, where it runs better.
Dennis Couzin
Berlin, Germany