Well any ad or pop up blocker or a browser setting can cause it ... ghostery is just one example of a blocking program and gave lots of issues for users when Connect Modern was first released.
Maybe Firefox has been updated and has a setting incorrect?
Hmmm. Reverted to v.39 - no change. Blank screen.
FF updated itself to v.41 - sudenly everything appeared - so that's great - except that the tabs at the top right don't work. Only the menu items on the left. At least it works - and it's clearly a problem with the configuration of FF on my PC - but I don't have a clue what it might be.