In the meantime I can offer these work around in how to manage high memory usage:

• Firefox uses too much memory or CPU resources - How to fix

Also the Telementary data sent on the browsers can help improve the performance. There are also automated tests done to check performance of different platforms.

If you are interested in providing a website example this could help create a bug. With the websites, there are have these tools to test the performance of the profile as well: This will test the javascript engine on desktop: https://developer.mozilla.org/en-US/d.../Reporting_a_Performance_Problem

Are you sure YouTube is using the Flash player on that page? Usually in that case plugin-container.exe would appear in the Processes tab.

To determine which player is being used, try right-clicking it and checking the last line of the short context menu.

If YouTube is serving the HTML5 player and you want to go back to using Flash on YouTube, there are a couple of extensions for that specific purpose:

• https://addons.mozilla.org/firefox/addon/youtube-flash-video-player/
• https://addons.mozilla.org/firefox/addon/youtube-flash-player/