The idea is not bad.
But the terrible behavior, when the add-on simply turns off and stops working without indicating the reasons - is very annoying.
It is understood that it needs to be updated in this case.
What for? It still working fine! I dont need any updates!
It will be easier just to install STYLISH and add a style:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
@namespace html url("http://www.w3.org/1999/xhtml");
@namespace svg url("http://www.w3.org/2000/svg");
* {
font-size: 13px!important;
font-family:Microsoft Sans Serif !important;
}

and set
layout.css.devPixelsPerPx = 1