How to switch Youtube back to the old layout.

Edit: This solution where I explain how to change the cookie worked for over 2 years, but Youtube has disabled it. I found a new solution that I have added at the bottom.

The new version of youtube is horrible. Everything loads extremely slow and is laggy. If you jump through a bunch of different youtube channels, everything gets even worse. There was an option on the top of the page to go back to the old version, but Youtube recently removed it and once you delete your cookies, you can no longer go back.

These instructions will restore the old layout on all youtube pages until you delete your cookies, then you will have to do it again. There is probably an easier way to do this with a script or through Tampermonkey or Stylish, but I don't know how to do that. There may even be an add-on for this already, but I don't like running a bunch of add-ons.

Chrome

• Open a Youtube page
• Press Control+Shift+I (that is an i as in information)
• Application tab on top (may have to click the arrows to view more tabs)
• On the left, in Storage, expand "cookies"
• Click on the cookie for Youtube
• On the right you should see columns for "Name" and "Value"
• In the Name column there should be one called "PREF". If this is missing, do something on the Youtube page to create it. Just disable autoplay on a video or change the sort order if you are viewing someones videos in their profile
• For PREF, double click the Value to edit it and replace it with f6=8 or add it to what is already there with &f6=8
• Click off of the Value or press enter to save it and then refresh the page. All Youtube pages will now use the old layout

Firefox (Firefox is the same as Chrome, but getting to the page to edit the cookie is slightly different.)

• Open a Youtube page
• Press Control+Shift+I (that is an i as in information)
• This step is only needed the first time. Click the gear icon on the top right of the window that was opened to view the settings page. Put a check in the box for "Storage"
• Click on the "Storage" tab
• Expand "Cookies" on the left and click on the one for Youtube
• Follow Chrome steps above

You can experiment on Youtube with different settings and see what it adds to the Value field if you want. f6=8 should set it back to the old layout, but my browser that already had it set back was showing f6=40008. I don't know what the difference is, but they both work.

There is also something you can add to the end of the URL to make the current page use the old layout instead of editing the cookie, but you have to add this to the URL and reload the page every single time you go to a new page. This is not needed if you edit the Youtube cookie shown above.

The way to temporarily change an individual page is just to add &disable_polymer=1 to the end of a video URL. If you want to use the old layout on a page from a users profile, you need to add ?disable_polymer=1.

If you are interested in a Greasy Fork script to do this for you automatically, Andrewski linked one in his comment below.

New fix

The new solution is just to change the user agent of your browser to an old version. When youtube sees this and thinks you are on an old incompatible browser, it will load the old site.

Firefox
My current user agent shows Firefox 50.0 and I simply changed the two instances of 50.0 to 43.0.

In Firefox, type "about:config" in the address bar and click through the warning to get to the advanced settings
Right click, create a new string
Name it: general.useragent.override
Then for the value, enter a user agent that uses Firefox 43 or older to make youtube use the old site
This is what I am using: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:43.0) Gecko/20100101 Firefox/43.0
If you want to remove it, just right click on the setting you created (general.useragent.override) and click reset.

Edit: For some reason, setting the user agent to Firefox 43 only randomly works. Sometimes Youtube will load the old site and sometimes it will load the new site. Repeatedly clearing the browser cookies and loading a youtube page will randomly switch back and forth between the old and new site.

Edit 2: I saw someone suggesting to use "Mozilla/5.0 (compatible; Googlebot/2.1)". In my testing this always causes Youtube to use the old version, but it makes some sites display wrong, like google search. I found that if I just add "Googlebot/2.1" to the end of my correct user agent, sites will display correctly and Youtube always uses the old version. There is a big issue with using the googlebot user agent though and that is that some sites will completely block you.

The user agent that I am currently using and is working for me is:
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:50.0) Gecko/20100101 Firefox/50.0 Googlebot/2.1

Edit 3: It is still working, but is getting more complicated. See my comment here for more info.

Chrome
I haven't looked up how to permanently change the Chrome user agent yet, but I have found that the last Chrome version you can use that will force the old Youtube is version 53. I tested this in the developer tools, but that only works in the current tab and while you have the developer tools open.