Adobe Flash Player don't show videos on YouTube after some point - Black Screen only

Hello. I have a problem with Flash Player on YouTube. It has crashed many, many times so I decided to open this thread for let you know about it if you don't know already.

Now, I've got no crash reports or anything but it crashes after some point. Even if it doesn't give me crash report, I understand it because YouTube videos don't run and I got only black screen. When I tried to close (click "X" at right above) Firefox, it denies to close with some click sounds. I can only close with combination of CTRL+ALT+DELETE and terminate process. After I closed Firefox and open new page, it works fine until I get the same situation.

By the way, I have laptop with these specs:

Intel Core i5 3210m

AMD Radeon 7670m + Intel HD 4000 ( with AMD Switchable Graphics called Enduro )

8 GB Ram

Windows 7 Ultimate 64 bit

I don't know what problem is causing it so I couldn't give you much information. But if you need any information, I'll try to help. Thanks in advance.

P.S: I'm sorry for the bad English.