assuming they are indeed open-pollinated fruit, yes you can save seeds
from them, but there's no way of knowing that they didn't cross-pollinate and will come true from the saved seed
. wouldn't hurt to try though if you like them that much. there are hundreds of commercially available varieties though that you can be relatively sure will grow true.
to save the seed
, cut the tomato in half(around it's equator) and squeeze the seed
and gel into a container. stir it every day or so if you want. after about a week a fungal mat will develop on top at which time just rinse all of the mold, etc out of the container. then set the seeds
on a paper towel or paper plate to dry for a week. thats it. then just make sure to store them DRY! they'll last for years if stored correctly