Windows 10 with Firefox 40.0.3 installed seems to be having major performance issues with Flash based websites and content. Sites that use Flash to stream video either show a black box or nothing at all, other sites load and then firefox cpu usage jumps to over 30% on my well spec'd PC well the page attempts to load and scrolling becomes very erractic and looking at the tab the loading progress circle becomes very choppy.

I have removed Firefox and tried in safe mode and with all addon's disabled and issue persists. Rolled back to the last 39 version and issue seems to be there too but moving up to the latest 41 beta seems to resolve issue.