I like it how Firefox ships with Open Sans, but I don't like the rounded look of CJK characters – I prefer the standard CJK font of Android 4.x. How can I change Firefox into using those fonts instead?
You can change the fonts used in content to your own choosing albeit only through advanced about:config preferences. This is not recommended.
Change these in about:config;
I've never tried changing these so you are at your own risk here.