Ok I finally solved it myself. It was a line in the .htaccess file that I didn't for some reason that was trying to explicitly define a php version that does not match any on this server (7.1).
Please mark this as closed and yes...triple and quadruple checking is sometimes necessary.
I was moving content from another server and upon trying to load any page my Firefox (also Edge and IE do the same) wants to download the file of type "PHP" instead of displaying it.
I thought maybe an .htaccess rule from the previous server was the issue as I needed some specific options once...
