Have you got the latest firmware on the router?
I would have thought of updating the firmware ,since I experienced the issue of firefox, but I wasn't sure which firmware version to use from the link below :
http://www.draytek.com.au/downloads.php?type=Vigor28...
But when you pointed to the firmware update , I decided it is a time to do that.
After updating, I can access the web interface by using both browsers Firefox and IE, locally and via VPN as well.
Before update I tried to use my laptop which is connected wirelessly to the router I was not able to access the web interface either from the Firefox or the IE, when I updated to the latest firmware the issue is fixed as well
firmware version before updating was 3.3.5.3
After updating firmware version was 3.3.7.2
Regards