Firmware updates are provided by many manufactures for many types of devices these days. Since most if not all Samsung TVs now have built in to their hardware USB ports and a place in the user level menu to do the update, and that the Firmware is published on their public web site, I cannot see how it would void your warranty. I do not own a Samsung DVD or Blu-ray, but, I am sure it also has an automated method for loading a downloaded firmware.
Looking at the executable files in the firmware update for my Samsung TV, it looks like to does some CRC checks on the firmware copied to the USB drive and checks that the hardware type is correct, as well as a few other safety steps, so, I think it would be very unlikely to have a firmware update go bad. Since my TV manual also includes steps for doing an update, as long as you follow the instructions for the update, it should be OK.
What WOULD void your warranty is if you downloaded a hacked or modified version of firmware off of a non Samsung web site, or attempted to install a firmware from another model of TV.
If your device cannot be updated without going into a service menu, then you may be in danger of voiding your warrenty. But, if all you need to do is go to a user menu (or put a disc in) and you are using a firmware image from a Samsung web site (public area), I would say you have nothing to worry about.
Please note that I do not work for Samsung. This is just my best guess based on how I read my Samsung warranty card. If you are really worried, then I would call Samsung (1-800-SAMSUNG) or see if Samsung_Tech replies.
Is it true updating firmware voids the warranty?

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