Both, I think. The rash of huge background videos is a prime culprit. Here's a bug I filed, for example: https://bugzilla.mozilla.org/show_bug.cgi?id=1197460

The Firefox Media Playback team will be tackling issues around multiple videos and background videos in Q1. For example, pausing videos until the user has viewed the tab (bug 1187778). Infinite-scrolling of Facebook's timeline page with multiple videos is another problem.

https://bugzilla.mozilla.org/show_bug.cgi?id=1187778

I hope we can push the Graphics team to enable hardware acceleration on Linux eventually, too. :)