I got several issues in the past with Firefox and shockwave flash (similar issues with i.e.9 that I don't use).
Since the version 11.3.300.257 installed on the 10 Jun I cannot watch embedded videos on youtube 'and some other web sites'.
When I desactivate the module
(tools - modules complementaires - select shockwave flash and desactivate)
shockwave flash 11.3.300.257 - version 10 Jun
everything is fine.
Unfortunatelly you cannot play online anymore or browse any flash website ....
This issue hapenned some time ago with green screen and firefox crashes ... it has been corrected but now coming back with new symptoms.
My configS: (there is some comments on ATI drivers on other forums but this is not my issue)
Win7 Entreprise - SP1 / x64
shockwave flash 11.3.300.257
Hope this helps other end users and provide some feedback to help to troublkeshoot this issue that seems to be reported widely on other forums.
Europe, Middle East and Africa