so I recently found out that you can use font.system.whitelist in TOR Browser (and soon in normal FF too).
You can see here the default TOR Browser prefs: CLICK
I downloaded a Windows TOR Browser Bundle for testing and after installing you can check in about:config, that those settings (like font.system.whitelist) do exist. In the file system they are for whatever reason not in prefs.js like you would expect, but embedded in the xul.dll file (at least when I search for some strings from the prefs you will only find them in xul.dll).
Now comes the problem with the Browser in whonix (v6.0.8) - it lacks for example the font.system.whitelist pref.
My linux knowledge is limited so I could not find where the binary for tor browser are nor what is used instead of xul.dll or how the prefs system works there, but it seems that it’s either a problem within whonix, that updating the browser does not add those prefs or a problem with the tor browser itself, due to updating through several versions where this value (font.system.whitelist) probably did not exist.
When you go into about:config, do you have font.system.whitelist?