Update

Since I moved back to Firefox (a few months ago when Quantum was in beta), I'm now using the following Add-Ons:

Enhancer for YouTube

This Add-On for Firefox combines both the MouseControls and ImprovedTube i used to need when working with Vivaldi/Chrome. Absolutely brilliant and a must have!

https://addons.mozilla.org/en-US/firefox/addon/enhancer-for-youtube/
https://www.mrfdev.com/enhancer-for-youtube

Ghostery

Smart Blocking of Trackers and Ads combined with a nice UI. Privacy 4tw!

https://addons.mozilla.org/en-US/firefox/addon/ghostery/
https://www.ghostery.com/

Stylus

Make pages look the way you like!
Stylus uses styles from userstyles.org to apply custom CSS for selected pages on the fly.

https://addons.mozilla.org/en-US/firefox/addon/styl-us/
https://add0n.com/stylus.html

New Tab Override

Very useful Add-On, especially when coming from Vivaldi/Chrome. Unfortunately there is a Bug in Firefox ^[1] which is still not addressed and basically makes the Add-On useless again (!).

https://addons.mozilla.org/de/firefox/addon/new-tab-override/
https://www.soeren-hentzschel.at/firefox-webextensions/new-tab-override/

YouTube Dark

As I dislike bright and dazzling interfaces the first thing I usually look for in any new app or website I'm using is a dark theme. YouTube was one of the first pages I kept looking for a dark theme.

These days we just need to click the profile picture, select 'Dark Theme' and turn it on. If you don't see the new theme of YouTube yet, open http://www.youtube.com/new once to switch to the new design.

Before YouTube offically enabled the dark theme option this was the way to enable the feature using a little hack involving Cookies:

Open the developer console like this:
Hit F12 or right-click "Inspect" any element -> Developer Mode -> Console
Paste the following into the console:

var cookieDate = new Date();
cookieDate.setFullYear(cookieDate.getFullYear( ) + 1);
document.cookie="VISITOR_INFO1_LIVE=fPQ4jCL6EiE; expires=" + cookieDate.toGMTString( ) + "; path=/";[^1]

^[2]

Once the cookie is created refresh the page by hitting F5.

Old Article on Vivaldi/Chrome

Adjust Volume using MouseWheel

I always liked the way you could turn the volume in a video up or down using the mouse wheel in VLC. To get the same behavior in YouTube, simply install the browser extension 'YouTube(TM) Mouse Controls' in Vivaldi/Chrome:

https://chrome.google.com/webstore/detail/youtubetm-mouse-controls/odhhfodnkaidhocaaaokdofhkoiahpbn

Tons of Options

'ImprovedTube' can automate a lot of tasks you usually have to do by hand after starting a video like:

• Changing quality to HD
• Switch to cinema mode
• Enable annotations
• Set playback speed

and many more! Get it at:

https://chrome.google.com/webstore/detail/improvedtube-youtube-exte/bnomihfieiccainjcjblhegjgglakjdd