Use medium weight Fira Font

The FiraSans package uses mdweight to set regular text, so calling \textmd explicitly has no effect whatsoever. If you want to use a specific weight explicitly, you can use one of the commands defined in FiraSans.sty:

• \firathin
• \firaultralight
• \firaextralight
• \firalight
• \firabook
• \firamedium
• \firasemibold
• \firaextrabold
• \firaheavy

Example:

\documentclass{article}
\usepackage[sfdefault]{FiraSans}

\begin{document}

Hello {\firathin World}.

Hello {\firaultralight World}.

Hello {\firaextralight World}.

Hello {\firalight World}.

Hello {\firabook World}.

Hello {\firamedium World}.

Hello {\firasemibold World}.

Hello {\firaextrabold World}.

Hello {\firaheavy World}.

\end{document}

If you can switch to either XeLaTeX or LuaLaTeX to compile your document, you could download the OpenType version of the FiraSans font family, which features 16 [!!] separate font weights. In comparison, the FiraSans package (which works with pdfLaTeX) offers only 9 font weights.

As may be seen from the following screenshot, the weight difference between "Medium" and "SemiBold" is rather subtle. To avoid any uncertainty among your readers as to what constitutes "bold", it's a good idea to emply a weight that's 2 or even 3 steps heavier than the chosen default font weight. E.g., if "Fira Sans Book" were chosen as the base font weight, then either "Fira Sans Medium" or "Fira Sans SemiBold" would be suitable candidates for the 'bold' font variant. (What's selected as the "regular" font weight for a document does not have to be the same as the weight that's labelled "Regular" by the font designer.)

Thus, one might write

\setsansfont{Fira Sans Book}[%
     ItalicFont     = {Fira Sans Book Italic},
     BoldFont       = {Fira Sans SemiBold},
     BoldItalicFont = {Fira Sans SemiBold Italic}]

to choose font weights "9" and "12" from the following list as the 'regular' and 'bold' font weights of one's document.

\documentclass{article}
\usepackage{fontspec}
\defaultfontfeatures{Ligatures={TeX,Common,Rare}}
\setmainfont{Fira Sans Regular}  % set as default

\newfontfamily{\FSTwo}{Fira Sans Two}         % 1
\newfontfamily{\FSFour}{Fira Sans Four}       % 2
\newfontfamily{\FSEight}{Fira Sans Eight}     % 3
\newfontfamily{\FSHair}{Fira Sans Hair}       % 4
\newfontfamily{\FSThin}{Fira Sans Thin}       % 5
\newfontfamily{\FSUltraLight}{Fira Sans UltraLight} % 6
\newfontfamily{\FSExtraLight}{Fira Sans ExtraLight} % 7
\newfontfamily{\FSLight}{Fira Sans Light}     % 8
\newfontfamily{\FSBook}{Fira Sans Book}       % 9
\newfontfamily{\FSRegular}{Fira Sans Regular} % 10
\newfontfamily{\FSMedium}{Fira Sans Medium}   % 11
\newfontfamily{\FSSemiBold}{Fira Sans SemiBold}   % 12
\newfontfamily{\FSBold}{Fira Sans Bold}           % 13
\newfontfamily{\FSExtraBold}{Fira Sans ExtraBold} % 14
\newfontfamily{\FSHeavy}{Fira Sans Heavy}     % 15
\newfontfamily{\FSUltra}{Fira Sans Ultra}     % 16

\begin{document}
\obeylines  % just for this example
{\FSTwo Hello World --- Two 1}
{\FSFour Hello World --- Four 2}
{\FSEight Hello World --- Eight 3}
{\FSHair Hello World  --- Hair 4}
{\FSThin Hello World --- Thin 5}
{\FSUltraLight Hello World --- UltraLight 6}
{\FSExtraLight Hello World --- ExtraLight 7}
{\FSLight Hello World --- Light 8}
{\FSBook Hello World  --- Book 9}
{\FSRegular Hello World --- Regular 10}
{\FSMedium Hello World --- Medium 11}
{\FSSemiBold Hello World --- SemiBold 12}
{\FSBold Hello World --- Bold 13}
{\FSExtraBold Hello World --- ExtraBold 14}
{\FSHeavy Hello World --- Heavy 15}
{\FSUltra Hello World --- Ultra 16}
\end{document}