I find the Media Viewer very helpful and time saving when viewing a number of images in succession.
But it frequently happens that Media Viewer does not open when I click on an image thumb, even though it is set to do so in the "appearences" tab of the preferences (by default on all or most Wikiprojects?) and I have never changed this setting. It happens less often in the English Wikipedia but I often notice this behaviour in the German Wikipedia as well as recently on Wikimedia Commons and the Portugese Wikipedia.
I am mostly working with Linux Mint 18 and the latest Firefox browser but this does happen on other setups as well.
Any ideas what is wrong?