I'll nominate Julian Seward, who among other things created both bzip2 and Valgrind, wrote parts of GHC (the Glasgow Haskell Compiler), and now works at Mozilla

I'll nominate Julian Seward, who among other things created both bzip2 and Valgrind, wrote parts of GHC (the Glasgow Haskell Compiler), and now works at Mozilla building and applying various analysis tools to Firefox/Gecko code.