If YouTube seems too slow on Firefox, this is quick fix
This is relatively old, but people still don't know about it. To fix it you type about:config in new tab, right click on any setting and select New>String. Then copy/paste these:
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:41.0) Gecko/20100101 Firefox/41.0
When you go to YouTube old format should appear and load much faster. For dark theme you can use Stylus addon.
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