Just a long shot here but it could be the Temp Google Chrome "bug".
What happens when you have a writable volume is that the disk is mounted into the operating system. If the writable volume doesn't have a drive letter it will see this disk as a network drive and say that it is unsafe and won't use it in any way. Maybe Firefox has the exact same issue.
You need the following registry key. What is does is give your writable volume a drive letter and hide it from the browser.
HKLM\System\CurrentcontrolSet\services\svdriver\Parameters\
DWORD = DriveletterSetting=6
For testing purposes you could off course just assign a drive letter to your writable volume. Then you won't need to add this key and test it manually.