URL: https://www.youtube.com/watch?v=xHvVhhFvLtY

Browser / Version: Firefox 57.0
Operating System: Mac OS X 10.13
Tested Another Browser: No

Problem type: Something else
Description: the macOS dock is hidden in the video fullscreen function, but the space is not used.
Steps to Reproduce:
The problem occurs when the browser window is not maximized. You have simply to use the fullscreen function for example on YouTube.

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