I installed Firefox 53.0.3 (64-bit) and Chromium 57.0.2987.98 (from the Debian repository).
Both do not have the font "Fira Sans". The text on the slides is not visible.
In the end I fixed it for me by removing the 'font-family:"Fira Sans",Helvetica,sans-serif;' line via the developer editor.
I installed Firefox 53.0.3 (64-bit) and Chromium 57.0.2987.98 (from the Debian repository).
Both do not have the font "Fira Sans". The text on the slides is not visible.
In the end I fixed it for me by removing the 'font-family:"Fira Sans",Helvetica,sans-serif;' line via the developer editor.