First try to increase the DPI settings. If it works, you are done. (~/.trinity/share/config/startupconfig: kcmfonts_general_forcefontdpi="96")
yeah, that's already set.
Out of couriosity: what value did you put there and did it change "something"?
it's been 96.
I have tried something else (I believe '120', not sure why I picked that) and it improved some things and made other things worse - had me jumping from config file to config file. somehow I've achieved equilibrium with 96.
and Firefox is a demon! ended up writing userChrome.css rules but strangely Firefox-esr (68.3.0esr) seems to honor them but Firefox 71.0 seems to ignore them. (I did the magic of trying to force it to read the file, etc.)
Firefox uses GTK3, and that's apestilence on it's own.