How to set the font style back again?

Question

Normally I just sue T1-fontenc and all is done. But for my frontpage I used FiraSans. Now I don't know how to, for the rest of the document, get back to the original font?

\documentclass[10pt,a4paper]{memoir}
\usepackage[utf8]{inputenc}
\renewcommand*\oldstylenums[1]{{\firaoldstyle #1}}
\begin{document}
THIS IS THE FRONT PAGE
\pagebreak
HERE I WANT A TEXT WITHOUT SANS-SERIF
\end{document}

Answer 1

Don't invoke the sfdefault option of FiraSans package. Just invoke \sffamily when you want that sans style, and \rmfamily when you are done.

If you need, additionally, to ditch FiraSans altogether after the front page and restore the, for example, Computer Modern sans font, just issue \renewcommand\sfdefault{cmss} and cmss will become the new sans font.

\documentclass[10pt,a4paper]{memoir}
\usepackage[utf8]{inputenc}
\renewcommand*\oldstylenums[1]{{\firaoldstyle #1}}
 %% option 'sfdefault' activates Fira Sans as the default text font 
\usepackage{FiraSans}
\usepackage[T1]{fontenc} 
\begin{document}
\sffamily THIS IS THE FRONT PAGE 123456789

\rmfamily HERE I WANT A TEXT WITHOUT SANS-SERIF 123456789

\renewcommand\sfdefault{cmss}

\sffamily FiraSans gone cmss restored 123456789

\end{document}