This morning was the last day for Oracle Forms in Firefox, I thought...
At 9:00 AM I had no problems starting my Forms-Runtime. Firefox 51.0 was the actual version at that point.
After lunch and restarting my Firefox I was running 52.0 with the new feature "no JAVA please" :-)
Very bad is, that firefox didn't give you any hint, when starting a Forms-URL. The only thing you see is an empty screen.
If you have this problem, here is the workaround:
Start the URL "about:config" in your Firefox. After the message, that from now on you are responsible for all things you are doing in the settings, you see all Firefox config-parameters.
Here you have to right mouse click and use "New"-"Boolean"
The name of the new parameter is: plugin.load_flash_only
The value of the new parameter is: false
Now you have a new Firefox-parameter, which lets you run Oracle Forms as before!