For this to work easily, you need to install the fonts for your system. Whether the actual fonts are installed by the TeX distribution is not, just in itself, very relevant. (Installing them this way may get you useful helper files, but will not make them easily accessible.)
However, on Windows, this is done automatically when the fonts are installed by MikTeX or TeX Live. It may also be automatic if you use your GNU/Linux distro's packaged TeX Live, rather than upstream's. For an upstream install, TeX Live provides a file which can be used to enable this for GNU/Linux.
This means that you do not need to use file names to access the fonts, but can just use the font names. Moreover, most features and font-switches will work out-of-the-box, at least insofar as they are supported by standard NFSS.
\itshape Fira Sans Italic
\bfseries Fira Sans Italic Bold
\upshape Fira Sans Bold
\scshape Fira Sans Bold Small-Caps
\itshape Fira Sans Bold Italic Small-Caps
\mdseries Fira Sans Italic Small-Caps
\upshape Fira Sans Small-Caps
\bfseries Fira Mono Bold
I do not seem to have any italic, oblique or other shapes for Fira Mono. However, many more weights are available for Fira Sans, which also has a Condensed version, and one further weight for Fira Mono. See the
fontspec manual for details if you want to access these or additional features.