Another strangly configured variable in Firefox…
To correctly display vertical traditionnal mongol writing in Firefox, go to the about:config URL, then change the value of the variable to 87 (that’s strangly -1 by default, this is only used for mongol fonts):
gfx.font_rendering.harfbuzz.scripts .
Update:, firefox 29 has 255 instead of -1 (looks like a change in the (computer data type from int to uint)
A specialized javascript site to write traditionnal mongol writing : http://www.unsen-sambar.com/
Thanks to Ganbold from #linux-sunxi for this tips.