Problems with edx course pages/videos

Question

Problem: Videos on the course pages appear randomly with or without Closed Captions (CC)
activated. If CC appears activated, then everything functions normally. If CC appears
not activated, then the video area expands (normal) but has a light-colored background
and does not start playing (the start/pause buttons appear, even change, but have
no effect)
Browser:
Using Firefox 24.0a1 and 24.0b5 (64bit): Random behaviour. Impossible to play. Tested with
FF 16.xx (32bit), 17.xx (64bit) too.
Test:
Go to edx.org, "View all courses", select 'edx Demo' (3 or 4 down), click 'register'
You'll have to register to edx.org before being able to enter the course (free).
Then, go to Week1, Lesson1, and click any of the videos ('envelopes' at the top).
Here, the page will block, can't select other option, and the video does not start.
Temporarily switching to "Homework" and back enables the video.
Note: I had to edit the "Troubleshooting" info below, as it exceeded the 30,000 limit.
I eliminated variables starting with 'print', which I suspect are not relevant.

Answer 1

You Flash plugin is out-of-date and needs to be updated. To do this, go to the [http://mozilla.com/plugincheck Mozilla Plugin Check site].
Once you're there, the site will check if all your plugins have the latest versions.
If you see plugins in the list that have a yellow ''Update'' button or a red ''Update now'' button, please update these immediately.
To do so, please click each red or yellow button. Then you should see a site that allows you to download the latest version. Double-click the downloaded file to start the installation and follow the steps mentioned in the installation procedure.
If the problem continues, please post below.