i know what you mean, the microb gui is not exactly intuitive for this. what i did was close all microb windows and open x-terminal. then

$ killall browser
$ find . -name prefs.js

make a back-up copy of prefs.js, just in case
edit prefs.js to insert the two lines
save prefs.js
open microb, look at about:config to make sure the two directives have been recognized correctly