I encounter a similar problem since the Quantum update. As soon as the developer console opens, the website displays additional content at the end (it disappears when the developer tools are closed), and an error message is displayed (“the debugger was stopped”).
As this problem occurs on my local development server, I am currently unable to give a test URL to reproduce the problem.
Here’s a screenshot displaying the rror more in detail.
Here’s a demo URL:
Load this normally, and the slider will work fine.
Open the developer tools, and reload the page. Neither the dev tools will work any more, nor the sliders, nor the F5 button.
Using Firefox 57.0 (64 bit) in German on Win10. Unfortunately, I am unable to install the nightly built on this PC. But maybe the demo URL allows you such a test.