After building the fonts (like to use the bold version of the font) following "Building from source" (without the patch as scripts/axes.py.diff is non-existent),
square roots containing fractions are not displayed correctly, if using lualatex.
\documentclass{article}
\usepackage{fontspec}
\usepackage{unicode-math}
\setsansfont{Fira Sans}
\setmathfont{Fira Math}[version=sans]
\begin{document}
\sffamily
\mathversion{sans}
inline math: $\sqrt{a}$ $\sqrt{\frac{a}{b}}$.
display math:
\begin{displaymath}
\sqrt{a}\sqrt{\frac{a}{b}}
\end{displaymath}
\end{document}
