In the good old days there was a way to resolve this in the settings of the Mozilla browser (Now Seamonkey) and set it to 96 dpi.
Nowadays this has to be done with a keystroke in the address bar of Firefox.
Key in :
in the filter of the about:config page key in:
double click the layout.css.dpi, the pop-up that appears can be set it to 96, and if thats still too small, to 120 or whatever your preference is, but rendering of sites might not be proper anymore.
In this way this small font size issue is fixed, without getting strangely rendered websites, due to manually setting the minimum fontsize, from “none” to for example “12”, what can cause problems with, for example, filling in forms.
Time to get my eyes checked,