i have no idea what you did ... but ill tell you what i did and you can take it from there...
*Firsts start the session, mostly like you did
nohup startx -- `which Xvfb` :99 -screen 0 1680x1024x24 & sleep 7
&sleep 7 , is so the session will start and you can still use the same session.
* Start Firefox
DISPLAY=:99 firefox & sleep 7
This will start firefox, you may have to kill the firefox in this session in order to open the other one.
* Confirm it is up and working
DISPLAY=:99 import -window root ~/Desktop/screenshot.png
You may need to install magis.... something to take the screenshot.
So startx, open firefox, and take screenshot ...
havent found the way to navigate in x99 using firefox .