It doesn't really matter where it sends it really. I'm going to look around to check, but I think that is just Mozilla letting you know that the software works fine in 64-bit Windows. Even if it is native 64-bit, as long as it runs, what is the issue?
EDIT: I was right. There is no official build of firefox that is 64-bit. There are a few floating around the web that random people have made, but that it it. When it says it supports 64-bit, it means it should run with no problems in a 64-bit environment. Thus, it goes to Programs (x86).
If you have anymore questions, feel free to ask.