Found a gem for YouTube (V3 extension: https://addons.mozilla.org/en-US/firefox/addon/v3-get-old-youtube-layout/) — which brings the old interface (from the start of XXI century) back. It is more clean, if it is even possible to say "clean" about YouTube interface, and also it is faster as hell than the "modern" one.
On the my box (Thinkpad X220 with i7, 16 Gb of RAM) the usual YouTube lags as hell, even while I'm scrolling. The old interface — works almost instantly, lol.
2 media
