xbindkeys, xvkbd, and Firefox

I'm trying to map alt+j to the down key and it's working everywhere but Firefox. In ~/.xbindkeysrc:

"xvkbd -xsendevent -text "\[Down]""
  alt + j

then with xbindkeys running, alt+j has various effects in Firefox depending on the context. In the address bar, where pressing the down key would select the next auto-complete option, the address bar flashes and the cursor goes to the end. On most pages it scrolls down as normal. In text boxes it goes to the next line down. In Google Docs the page scrolls down instead of the cursor going to the next line. According to https://www.toptal.com/developers/keycode the down key is being received in JS but with the shift key briefly pressed first. Any suggestions on how to debug the issue or ideas for alternative ways to remap key combinations that works in all applications would be appreciated.