I have updated firefox & flash, downgraded flash to 10.3 and tried everything but videos still appear as a black screen after months on my macbook running Lion.

I have had this problem for months now and in the past - usually though upgrading/downgrading and googling I can get videos working again but this time I cannot! Videos will load in chrome and safari and the occasional video will load - such an embedded links from putlocker etc. for streaming. I mostly notice the problem with youtube because nothing will load on there. I just don't know how to fix it now - I have updated firefox again, updated realplayer, updated and downgraded flash... A solution would be highly appreciated!