Simpler, sure. Safer and more secure, how? There's been a lot of new security features in Firefox recently that you'd be missing out on if you used something that old. You can't put "just" in front of "execute JS" (or "render HTML" for that matter); that's a pretty complex task with a lot of security concerns. I can't imagine that the communities of these Firefox forks can keep up with backporting upstream security fixes, especially since the modern Firefox codebase has diverged so much.