I have no idea. Probably just a domestic mix, like most. Hopefully someone that does will come along soon, but just had to say, "WOW, look at that tail!!"
She is a gorgeous cat and beautifully groomed. Color / pattern looks like a Silver Torbie. I agree with Stefan; the cat does look more like a Siberian than other breeds, but I have never seen one with a tail like that .
It's likely that the tail looks bigger than it is , since sometimes in photographs the camera can really distort size and make parts that are closer appear larger. But even allowing for camera distortion, that tail is more than I have ever seen on a Siberian.
It's also possible it could be photoshopped to make the tail look bigger. but I would still guess the original had a very very big fluffy tail.
A big tail is more like a Maine Coon but at least in this photo the head doesn't look very typey for a MC.
I am wondering if there may be some Persian ancestry, since that's the main breed where there has been some serious selective breeding for super long hair on the tail.
it's possible she may be a mix from cats who look sort of Siberian like , with some Persian in there. Siberians are a breed that initially just developed on their own by random breeding and adapting to the environment, so other cats may have a similar look if their ancestors came from a similar environment.