"First to file" just means that if two patent applications come in at around the same time, there won't be a court case with a bunch of discovery concerning who was working on the actual invention first. Its just the first patent request into the office now. This does not affect prior art. If anything it makes it stronger by preventing the claim that a so called inventor was working on an idea prior to the disclosure of the prior art.
The fact that a two sentence blog post about using a simple 3 way switch to control a microphone is in fact prior art strong enough to bust a potential patent is just a sad, sad commentary on the quality of patents being issued today.
In the US, the rule used to be first to file, but is now first to invent (along with the rest of the world) since the enactment of the America Invents Act in 2013.
First to file/invent isn't actually relevant for prior art though. Any publicly available source that describes the invention can serve as prior art. However, patent lawyers are very good at "writing around" existing patents and other prior art.
Actually, you have it backwards. The US was previously a first-to-invent country but as of March 2013 is now first-to-file. This brings the US in line with the rest of the modern world, which is already mostly first-to-file.
