I Don't Care About Cookies: Removes idiotic GDPR cookie notices introduced from the top to the bottom by the clueless beareaucrats at the EU.
Tab Mix Plus: Multi-row tabs, custom css backgrounds and fonts for tabs based on if they are loaded or not. Adds Open in Foreground to left-click link context menu. It also does a whole lot of other stuff I don't care about.
Pentadactyl: Adds vim-like keybindings for keyboard based navigation except on some sites like youtube I don't really use the mouse much.
Flow Youtube Chat: Flows youtube chat on the screen of youtube livestreams like on niconico. Saves you from the effort of having to look at the chat and then at the video back and forth. More importantly it looks cool.
Stylem: Allows you to add css stylesheets to websites to load on your browser. I use it to style the quickdial page of the browser.