I think the poster is making a distinction between Firefox and Mozilla founders, but it could be more clear. Mitchell, a Mozilla founder, is still around. The Firefox founders, who are Dave Hyatt and Blake Ross, and possibly Joe Hewitt (https://en.wikipedia.org/wiki/Firefox_version_history only states the first two, https://en.wikipedia.org/wiki/Firefox#History all three), have not been at Mozilla since the early 2000s.