Fix Firefox YouTube default Flash mode

**Do not merge!**

**Description**

Video speeds .5, 1.50 and 2.0  not available in Firefox

The only video speed available in live courses is 1.0 in Firefox. I was able to reproduce in the following courses when using Firefox. The issue does not occur in Chrome.

https://courses.edx.org/courses/DelftX/AE1110x/1T2014/courseware/669e9115be17498a9fa0cc5cee0cb78f/3ca3d5812aef405a8aaa1f296d4985aa/

https://courses.edx.org/courses/DelftX/AE1110x/1T2014/courseware/669e9115be17498a9fa0cc5cee0cb78f/3ca3d5812aef405a8aaa1f296d4985aa/

**Jira issue**

[BLD-895]