xulrunner is more than just javascript, it is also XUL and XPCOM, and is meant for apps with a GUI. This appears to let you run just JavaScript as if it were in a "virtual machine"-ish environment.
how do they plan on growing your team? do they have a management strategy? may seem mundane, but it will end up affecting your job directly, and if they don't know what they are going to do, you may end up having to deal with a lot of the fallout of disorganization.