What is the current user-agent string(s) of the Tor browser?
What user-agent strings has the Tor browser (and older Tor software) used in the past?
Tor browser user-agent strings at this point in time Dec-31-2014 for the current version is:
Mozilla/5.0 (Windows NT 6.1; rv:31.0) Gecko/20100101 Firefox/31.0
In the past Tor browser has used many different user-agent strings, depending on which version of Tor browser you are interested in, one can search for that particular user-agent string.