Native 64 bits flash plugin is still in an alpha state according to Adobe, and it crashes a lot (for e.g. when I load Gmail, the flash stuff in Google Talk lead Firefox to crash) on my main computer, even with a whole fresh install (Ubuntu 9.10 or 10.04 up-to-date, nVidia proprietary drivers, GeForce 8600M GT). So I don't think that using the native 64 bits plugin would be a good thing for now.
Native 64 bits flash plugin is still in an alpha state according to Adobe, and it crashes a lot (for e.g. when I load Gmail, the flash stuff in Google Talk lead Firefox to crash) on my main computer, even with a whole fresh install (Ubuntu 9.10 or 10.04 up-to-date, nVidia proprietary drivers, GeForce 8600M GT). So I don't think that using the native 64 bits plugin would be a good thing for now.