Evenafter applying the offered solutions for "pygmy" font displays no result
The logical assumption is that one can set
(a) a fixed font size in FFox
or (b) at least a minimum display size (esp. in popups where a sans serif font could/should be separately specified and used) and not conflict with a larger general page display font. (b)
For people with presbyopia (old age ruined eyesight) life is already tough enough with 6pt type used to hide the asterisked conditions....
It is BTW - a strictly FFox problem AFAIK
Wšykne wótegrona (1)
It is probably better to use Firefox's Zoom feature than to set a minimum font size.
The Firefox Page Zoom feature does a domain by domain level of saving the users preferred zoom level settings, there is no default Page Zoom level setting in Firefox, as with some other browsers.
Try the Default FullZoom Level extension and set a default zoom level:
https://addons.mozilla.org/en-US/firefox/addon/6965
Or try the NoSquint extension:
https://addons.mozilla.org/en-US/firefox/addon/2592/