...
Go to the Microsoft Visio Viewer download site and install the viewer directly.
Restart IE (This is an important step! Do not skip doing this).
Test on IE with the Visio viewer test (the test is not dependent on the Visio for Confluence Confluence app). If it works here, then it must work in Confluence also. As the test works with IE, it would also work in FireFox IE tab and Chrome IE tab extensions.
...