The OP is from 2010. For those wondering what sort of user-agent a brand-new browser engine would adopt in this era, see this discussion regarding inventing a UA for Servo, which involved collecting data from popular sites in the wild to see how they treat UAs: https://github.com/servo/servo/issues/4331
TL;DR: you can see end result for each platform here: https://github.com/servo/servo/blob/2d3771daab84709a6152c9b5..., and it looks like "Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:55.0) Servo/1.0 Firefox/55.0"