Flash uses hardware acceleration, for some people the hardware acceleration has caused display issues with Flash content. To see if this is the case for you, try turning off the hardware acceleration as described in this article - Flash videos won't play full screen

For me blocking YouTube cookies solved this problem.

If you can still click the blank video area and go to the video on the YouTube website, first delete any current YouTube cookies. Then go to Tools > Options > Privacy and set the following;

1. Firefox Will: Use custom settings for history

2. Check the box "Accept cookies from sites"

3. Keep Until: Ask me every time

4. Click OK

• Other setting are your personal choice?

Now each time you load a new site an options box will appear, offering you a choice between "Allow", "Allow for session", and "Deny."

The next time YouTube wants to set a cookie choose "Deny", and you should be good to view any YouTube videos on the web?

This method of cookie management allows you to curtail the number of active cookies for any given session. Rather than having countless active cookies for sites you are not browsing often this acts rather like a light switch, turning on and off the cookies you are actually using for each session (as I understand it). Whilst at the same time allowing you to set basic rules for different sites.

If you switch to this method, shorterm you will encounter the relevant pop-up often, and after a shorter time this will fall off once the bulk of the regular sites which you visit have been set.

Personally I only "Allow" sites which I trust 100%. For the bulk of websites I use "Allow for session", and "Deny" those as necessary.