In our slide presentations on opendreamkit.org, all slides but first and last appear white on firefox

[nthiery]

See e.g. page 2 of http://opendreamkit.org/activities/2016-09-27-egi-workshop-krakow/.

Firefox version: 50.0.2 on Ubuntu 16.04.

Works for me with chromium.

cc: @VivianePons, @defeo