Your QA / Dev environment should already be pointed to your Dev Launch file... all of your developers, QA, and new features (including issues with that code) shouldn't be sending data to your production suite.
You should have Dev Suite, configured into your Dev Launch Build, and all of that should be always set up on your Dev, QA, UAT, etc servers....
But if you don't have that, I don't know of any good extensions to do what you are wanting... the only thing that comes close (from what I am seeing) is using Local Overrides in your browser:
But I don't know if this will allow you to do what you need or not... Safari seems to be pretty light on available extensions.. You might be better off getting Chrome and using it to test, as you can use extensions such as "Launch Switch" (which allows you to choose the Launch Environment) or "Resource Override" that allow you to swap any resource for another:
I don't use Safari, but I couldn't find any equivalent extensions when trying to come up with a solution for you.