Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

Avast is known to cause problems. Disable it as a test.

check this:

• see if there are updates for your graphics drive drivers

https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)

https://forums.adobe.com/message/4468493#TemporaryWorkaround

• disable hardware acceleration in the Flash plugin

https://forums.adobe.com/thread/891337 See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

thank you for your reply, I did most ot those things already and the problem persists, the only thing I haven't done is disabling avast (need to find a replacement) and disabling protected mode (I'll actually do that in the afternoon)

however I noticed that my previous statement was wrong, while flash doesnt' crashes while using chrome I can't watch any youtube videos. The page loads and I can navigate the video using the slider, but it doesn't actually play. It only shows still images

Can it mean that the actual problem is related to windows or my hardware?

N7Shepard said

Can it mean that the actual problem is related to windows or my hardware?

Can't answer that until the issue is solved.