Comment 14 for bug 39447

It is a Mozilla (i.e. Gecko) problem, not a GNOME one. If you type about:config in a browser and filter on 'font.name.sans-serif' you'll see the fonts (for different scripts/encodings) Gecko uses when 'sans-serif' is asked for by an HTML page. The problem is it has Helvetica for some of them there when it should have sans-serif. Putting sans-serif in all of these solves the problem. You may wish to fix 'font.name.serif' and 'font.name.monospace' too (replacing Times and Courier with serif and monospace). All that looks like a remnant from Netscape's days.

PS Another bug: when you change an entry from Helvetica to sans-serif it may not change on the screen (i.e. you still see Helvetica), refresh the page to make sure it really changed.