Can't start tests

Trying to test a page tests run only with Chrome.

At Firefox test fails with an error Error starting Firefox: 'module' object has no attribute 'ensure_binary'

At MicrosoftEdge (EdgeHTML) tests fail with Error starting browser: Message: Unknown error

Test instances:
[list]
[] Dulles, VA - Thinkpad T430 - Firefox - 3GFast
[
] Dulles, VA - Thinkpad T430 - Microsoft Edge (EdgeHTML) - 3GFast
[/list]

What could this mean? I can’t really attribute the cause of these errors:
[list]
[] is it the tested website which reacts anyhow wrong?
[
] Or is it really the test engine, which has problems with this certain website?
[/list]