This will be a bit of rhetorical question, but ...
Assume for the sake of discussion that a robot, for reasons best known to itself, comes around with a fake humanoid UA--Firefox/11 or the like--but that this robot still wishes to obey robots.txt. (I did say it was a rhetorical question.) How would you identify said robot in robots.txt?
My first thought was "User-Agent: Mozilla" but it turns out that plenty of reputable* robots, up to and including the Googlebot, start their UA strings that way. At least one version of the mobile Googlebot calls itself Chrome. Would it work to say "Firefox" or possibly "Windows"? There do exist a handful of robots whose UA string says "Firefox" along with the robot name, but those are rare enough to handle on a case-by-case basis. Besides, I'm pretty sure I already block them.
Or is the very idea of a compliant liar so far-fetched that the answer must remain unknown and unknowable?
* I originally said "respectable", but decided that "reputable" allows more wiggle room.