Lighter version of Fira Math

I installed Fira font (sans) and Fira Math from the MikTeX console. There are numerous weights available for the text .

For Fire Math I found only one: Regular.

The closest weight for text so far is book, but ... keep reading.

% !TeX TS-program = xelatex 

documentclass[aspectratio=169]{beamer} 
usepackage{FiraSans}

usefonttheme{professionalfonts}    
usepackage[mathrm=sym]{unicode-math}
setmathfont{Fira Math Regular}

usetheme[titleformat=smallcaps,block=fill]{metropolis}
%metroset{outer/frametitleformat=smallcaps}
setbeamertemplate{navigation symbols}{}
setbeamertemplate{blocks}[rounded] 

begin{document}
        
begin{frame}{Frame}
        
textsf{The formula (regular)}

{firalight The formula (light)}

{firathin The formula(thin)}

$z = d - kappa x = (1 - kappaxi) d = zeta d$

{firabook  The formula closest (book)}

{firamedium Other formula (medium)}
    
end{frame}

end{document}

I found that the regular weight comes up as light! So perhaps this is the heaviness you notice in the math fonts.

At this time I do not know how the theme metropolis is handling this font but it looks that uses Fira Sans Light as the regular weight. For example using usetheme[titleformat=smallcaps,block=fill]{Dresden} the regular font comes out alright. Also there are less space between lines.

UPDATE

This setup will produce light weight text and formulas, like is shown in the demo for the metropolis theme.

% !TeX TS-program = xelatex 

documentclass[aspectratio=169]{beamer} 
usepackage{FiraSans}

%   usefonttheme{professionalfonts}    % do not use
%   usepackage[mathrm=sym]{unicode-math} % do not use

usetheme[titleformat=smallcaps,block=fill]{metropolis}
setbeamertemplate{navigation symbols}{}
setbeamertemplate{blocks}[rounded] 

begin{document}

begin{frame}{Frame}

    The formula 
    
    $z = d - kappa x = (1 - kappaxi) d = zeta d$    
    
    
    begin{equation*}
    e = lim_{nto infty} left(1 + frac{1}{n}right)^n
    end{equation*}

end{frame}

end{document}