When a tab with youtube is opened up and the tab is activated while my cache has saved some data, the CPU usage will go way higher than it normally does. It doesn't matter whether I watch a video or am only on the main site. Also, if I go onto any other site (of the ones I tested), the CPU will stay at normal levels. I currently have Windows 10, but that doesn't seem to be the problem. I recently started using Whatsapp on my browser. At first I only used Whatsapp, but later on I had two windows opened up, one being Whatsapp and one being Youtube, so I could chat and watch videos simultaneously. There didn't appear to be any problem with it. Of course my cache also has been filled with data then and also has been full. But since I updated firefox to the verison 56.0 I have these problems. Only restarting would stop it from hogging my CPU. Basically when I'm just on Youtube, everything is normal and there's no problem. But as soon as I open Whatsapp and chat a bit, even if I don't do it simoultenously, Youtube will hog the CPU afterwards and lag a bit. I tracked down the problem to the cache being somewhat full. I increased the number of how much MB it can use, but the problem is already there, even if the cache is not even full. I didn't experiment enough to be able to give numbers, but I can say, that around 100 MB is already too much. Which (of course) fills up pretty fast when using Whatsapp. It's interesting though, that it only happened after I updated to firefox 56.0. I haven't tried yet to fill up the cache using other sites and taking a look whether Youtube will start hogging CPU. Again, it only does this, when a tab containing any URL from the Youtube domain is activated. When I open up a new tab, the CPU will decrease back to it's normal value, also when I open another site it will revert to its normal state. But as soon as I open Youtube it will increase the CPU usage again. I'm not 100% sure, whether the cache is actually the problem, but it did seem obvious to me, as the high CPU usage slowly starts as soon as I fill my cache. So, summarized, I think there's some problem with the new update of firefox, something has been changed that should not have been changed. Also, to mention it and maybe speaking against my thought of the cache being the problem, when I empty the cache, Youtube will still use a lot of my CPU. Again, not only videos, but the whole domain does this. Only after I restart it will use less again, like it originally did. And yes, I did test using Youtube for a long time without using anything else to eliminate the possibility of the CPU usage being high because firefox has been open for a long time. But everything was fine. I hope somebody really knows something about this, as this problem is bugging me quite a lot and I only know the gist of it. Before posting this I tested one more thing which seems to be a partial solution and may help out to figure out the real solution: When I opened up any Youtube site before my cache was full, the CPU was still low. Then the cache slowly started to fill and said problem happened: the CPU usage raised to tremendous levels. And as I said, when I cleared the cache, the site still used too many ressources while it was open. But I just randomly stumbled upon my fan being quiet, thus using little CPU, while I actually was in the same session and on Youtube. So I found out, that if you reload the site after the cache has been cleared it will also revert to its normal CPU usage again. Clearing the cache without reloading the site on the other hand keeps your CPU high. Which is good to know, because if it gets too bad I can reload, but of course reloading a video every minute or so would be quite annoying. Anyways, that strengthens my previous thought of the cache being the problem in this case. The first image is a screenshot of the site before I reloaded it and the cache was pretty full, but I cleared it. Still, the CPU was as high as you could see (and the longer I wait before clearing the cache the higher it goes, in this case I cleared it pretty fast). The second image is a screenshot of the same site after I reloaded it after clearing the cache. As you can see, the CPU usage reverted to its (for my computer) normal levels. And I also tried using the safe mode to see whether one of my addons might be the problem. But the same thing still occured even while using the safe mode.