Install Fira Sans Font using Texlive 2012

I assume that you use XeLaTeX or luaLaTeX (which are necessary for fontspec).

If this is true, those programs must know which fonts are available on your system. For that you must typefc-cache in a terminal after the installation of the font to update the cache of fontconfig. To verify if the system is aware of the font, you can type, in a terminal something like :fc-list | grep -i fira which should list all the fonts whith "fira" in the name.

I don't know if fontconfig search a font in a personal texmf by default. If this is not the case, you could make a link from .fonts in your personal directory to the Fira directory and run fc-cache to update the cache of fontconfig.

In file beamerfontthememetropolis.sty from the theme, change the line

\setsansfont[BoldFont={Fira Sans}]{Fira Sans Light}

to

\setsansfont[BoldFont={Fira Sans},ItalicFont={Fira Sans Light Italic}]{Fira Sans Light}

I don't know why luaLaTeX need to specify the italic font explicitly, because XeLaTex compiles the demo file without any problem on my system (Windows+MikTeX).

In addition, you might also want to change

\defaultfontfeatures{Mapping=tex-text}

line to

\defaultfontfeatures{Ligatures=TeX}

to get the theme fully compatible with luaLaTeX.