Selenium - WebDriver::Error::WebDriverError: unable to obtain stable firefox connection in 60 seconds

I use Capybara for the integration testing of my application.

After updating my system to the new Ubuntu 16.04, I had to reinstall many of the gems of my application.

Generally the application run successfully, but the integration test has some problem.

Particularly, if a run a test, the browser-simulation does not automatically start anymore and the terminal reports the following:

Warning: Unable to check javascript error status.

Selenium::WebDriver::Error::WebDriverError: unable to obtain stable firefox connection in 60 seconds (127.0.0.1:7055)

Does anyone know how to solve this problem?