App runs fine on Windows Firefox 50 and other browsers on windows and mac, except on firefox for mac.
What I am seeing on Mac ( Firefox 49 and 50 ) . It loads the application, but shows a white screen. When I hover over the white screen, I can see the pointer changing to hand cursor for the buttons. When I click the buttons, they jump to next screen producing audios as programmed. But nothing is visible. Everything is white screen.
Anything in the console?
I have sent a privat message to you along with the working link, as it is not possible to share the client website link here publically 
By the way , sorry, I forgot to ask. How am i supposed to use -Dcanvas ? Is it a tag in project.xml or is it a compiler parameter, I will need to add in somewhere inside program settings ?
-D means it’s a define, so try <haxedef name="canvas" />.
You can also do it on the command-line, such as openfl test html5 -Dcanvas