Pretty sure the TOR user agent is just default firefox, by design. It's very easy to detect OS with very rudimentary fingerprinting techniques, a lot of which are blocked by the TOR browser but they can never get them all.
I'd expect TOR browser to mitigate this. Canvas2D is disabled for instance and system fonts aren't exposed.
Most other things could be mitigated by making every platform use the same code paths for e.g. font rendering. It should be pretty damn hard to determine which OS it is when the userspace is the same. I don't know whether TOR browser currently does this though.