Hi Konally!

I wasn't able to reproduce this issue on my side. I had 6+ videos open in separate tabs and didn't notice any lagging when I moused over the tabs, or within the videos themselves.

Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache. Note: This will temporarily log you out of all sites you're logged in to. To clear cache and cookies do the following:

1. (Firefox 57 and above) Click the Library button and choose History.
   (Click the menu button and choose History in older Firefox versions.)
2. In the History menu, choose "Clear Recent History...".
3. Under "Time range to clear", select "Everything".
4. Now, click the arrow next to Details to toggle the Details list active.
5. From the details list, check Cache and Cookies and uncheck everything else.
6. Now click the "Clear Now" button.

Further information can be found in the Delete browsing, search and download history on Firefox article.

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Thank you.

I've tried all of that before and recently, newly installed the OS. I also opened the windows before signing in to my profile. I was having the same problems with the last old install.

Okay what you have to do is, when all of the tabs are open wait a few seconds then quickly move the mouse pointer over to the first tab; I've done it many times and it lags. This does not happen with Brave or Edge