Comment 12 for bug 1579543

For Ubuntu 14.04:
From: http://askubuntu.com/questions/457461/gvfs-mount-not-working-on-ubuntu-14-04-for-wireless-hdd/459093#459093
Find the path to your network drive as it would be found in Terminal.

Now you can copy and paste this path into Firefox to access network share.
Make sure the share is mounted first. So, for me that path was:
/var/run/user/1000/gvfs/smb-share:server=192.168.1.1,share=router_drive/

Once you confirm you can access the network, go back to the gvfs folder and save your share as a bookmark. Now you have it in the file open menu.

I have Firefox version 52, so I guess no one is working on this issue.