I am using Firefox version 49.0.1 and can access MiVoice just fine.
I go to Users and devices, search for extension, click the link.
On the list of tabs, select keys and click clear all keys.
Then when I click Save changes get a pop up box that says "An error occurred window.showmodaldialog is not a function"
Tried on another device and get the same result. Beside rebooting phone system, is there another fix?