I've found a lot of threads about OSX and FF57 and excess CPU ... for me, it's made my computer basically unusable. But what I'm seeing, that none of the other threads have mentioned, is that simply hiding the application kills the CPU usage. So my average energy impact my be 100 if I leave the app visible with some windows running, but simply hiding the app causes that instantaneous impact to drop to 10 or 15. This would seem to be an issue with the rendering piece. I can verify this happens with or without hardware acceleration enabled (and I have tried all of the other suggested fixes, without luck, some of which have set my work back quite a bit). To help debug this problem, let us know if hiding the app affects CPU/energy usage.

Hello...thanks for your response. I am not too familiar with hiding an application, but from what I can tell, this is something that you do when you are not using an app. My problems occur when I am actually using Firefox i.e. loading a webpage, so I don't think I could hide it if I am trying to view something via the browser. Right?

Correct. But if you're looking for a quick fix, cmd-H (command key plus "H") will hide FF, and reduce CPU usage. Surprisingly, bringing FF back into the foreground (e.g. clicking on the FF icon in your dock) doesn't cause all those resources to be used again (in my case) ... I need to revisit a few of the hidden windows (which take a second to load) before I get CPU issues. This tells me that FF is doing something very non-standard on OSX with display, and is trying to use the same code across platforms for a task that should be platform-specific.

If you can confirm this is what happens for you, it might be a lead for the FF folks trying to fix this problem. I'd be curious if I'm the only one seeing this, or if this obvious lead has been overlooked by everyone dealing with this issue.