Opera 12s source is also available, yet it would still be illegal for someone to fork it as it'sa closed licence. Available source is not open source—the difference is having a legal right to continue development.
There have been very interesting creations, like 10 versions of Windows on one live DVD, made by those who couldn't care less about the legal aspects. Such a "underground" browser would be an interesting thing indeed... there was a discussion here on HN about the source code leak not long ago.
