Firefox 34 doesn't seem to play Flash videos on You Tube. The player element stays black. This is with both www-client/firefox-34.0.5 and www-client/firefox-bin-34.0.5. I'm still building www-client/firefox-34.0.5-r1 but judging from the change log I don't expect a relevant change. www-client/firefox-bin-31.3.0 on the other hand works as it should. Flash is www-plugins/adobe-flash-11.2.202.424.
Created attachment 391074 [details] emerge --info
Works fine here. Does the console log any interesting errors?
Sorry, the Flash Block addon was to blame, even after disabling it using its tool bar button. Disabling it on the Addons page made things work again, and also showed me that Flash Block is officially incompatible with this version of FF, except that it was still available due to my use of the Compatibility Reporter Addon. So it's my own fault, in a number of ways. Sorry for the noise!