1. Go to mobile version addons.mozilla.org
2. Pan right on one of the menu items
The menu will unexpectedly pan. We recently landed a bug to allow scrolling for overflow: auto and overflow: scroll elements, so you might want to set your div to overflow: hidden.
Thanks for filing this. Due to resource constraints we are closing bugs which we won't realistically be able to fix. If you have a patch that applies to this bug please reopen.
For more info see http://micropipes.com/blog/2014/09/24/the-great-add-on-bug-triage/