It's not switching to the old YouTube. Something's not right. Fix it pronto!

It works nicely for me, the only thing that I wished for was compatibility with dark mode! Oh well.

Seems to work well

It still work, very very well

Good

"This extension allows you to switch between old and new designs."

No it does not. :P

One of the best Add-ons for Firefox, anything that lets you get away from the new gigantic Youtube screen is a winner, many thanks to the Developer, easily 5/5. This even gives you the choice between the two so it's a win win.

EDIT:
1:43am GMT-5 August 17 2018
Problem with video window misaligned was fixed (seems youtube fixed itself)

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Really nice addon, an easier way without messing with cookies like this thing you paste in console after going inspect on youtube

document.cookie = document.cookie.split(' ').filter(o=>o.indexOf('PREF=')!==-1)[0].replace(';','')+'&f5=30030&f6=8;domain=.youtube.com;path=/';

Sadly on August 17th 2018 (basically most of the 16th day) there seems to be a youtube update that as the reviewer below, misaligned the video frame on the page, even with the copy paste from above you get the same misaligned video frame on firefox or chrome.

Let's just hope youtube fixes itself... or this extension or another edit the layout.

Helpful