Update building Firefox docs

MDN
#1

hey @gerladobarros,

I was reviewing the mdn docs for the Firefox build, and it could use some updating do you happen to know if anyone has plans to update the MDN guide docs. I copy and pasted the passage below from the docs. Specifically the link for Visual Studio Community 2015 Update 3 takes you to a VS 2017 Community edition build.

Visual Studio 2015

Official builds currently use Visual Studio 2015. To use it, download and install Visual Studio Community 2015 Update 3 from Microsoft.

thanks ~steve

My Firefox Nightly presentation
#2

@mhoye Any thoughts on this?

#3

here is a link to get the VS 2015 version.