How do I change the font size of the browser permanently?

I noticed it helps to modify this setting in about:config:
layout.css.devPixelsPerPx
A value of 1.7 seems to be pleasantly easy on the eyes (at least mine). But that is only temporary. As soon as I restart the browser, it's back to tiny fonts again.

However, I cannot seem to make that setting stick. The browser just ignores the changes to the pref.js I made. Adding a user.js file did not make a difference, either.