Stefan is known on the Internet as Costenslayer and has used this name for many years. As a result, its origins are now lost. He also helps with Mozilla Community web services, with hosting requests, as well as Community emails and much more.
After attending his first Mozilla festival back in 2013, he has taken part and volunteered ever since (current at time of posting). Here is his blog post on volunteering at MozFest 2014. He is overseeing the Youth Mozilla (YoMo) mentorship program, which inspires young people to contribute to the Mozilla mission, and use open source software. Follow how YoMo is growing on his blog. In addition to all of this, he is also one of the many Mozilla reps and you can see his Reps Profile.