It adds a list of the most recently closed tabs to the tab context menu

Tab Snooze - allows you to close a tab and have it reappear at a chosen time later

Media URL Timestamper - automatically inserts the current timestamp of the YouTube/Twitch video you’re watching and updates it in the history in case you accidentally close/navigate away from the page or go to a different time in the video

Feedbro - RSS reader with filtering capabilities

Redirector - auto-redirect specific URLs (for example, changing a YouTube Shorts url into a regular one, or changing Reddit links to always go to Old Reddit)

Undo Close Tab Button - allows you to restore recently closed tabs including the tab’s history in the back button (max amount = browser.sessionstore.max_tabs_undo)

Violentmonkey - using userscripts that allow you to change things on websites.

• For Instagram - unmuting videos + setting their default volume
• For YouTube - disabling the subtitles/captions + disabling “autoplay next” for playlists + disabling autoplay of channels homepage video
  • a way to disable specific keyboard shortcuts (you need to manually add the code as a new script). I use /^(Key)?(End|I|O)|(Digit|Numpad)\d$/ instead of /^(?:Digit|Numpad)\d$/ (thanks to this post), to also disable the End/I/O keys in addition to the number keys.
• Note that, at least for Violentmonkey, if the userscript doesn’t have the “://” part of the url in the @match line then you need to add it in the userscript settings after installing the script (for example, if the @match line of the script only has *.youtube.com/* then put *://*.youtube.com/* in the “@match rules” line in the settings)

YouTube Comment Reader - allows you to search through the comments of a video (by clicking on the addon in the Extension menu and then clicking on the “YouTube Comment Reader” at the top or the “X Comments” at the bottom of the tooltip)

Page Shadow - allows you to use dark and light themes on sites that don’t have the option to change it.

And if you’re like me and you find that some YT videos feel too slow but 1.25x is too fast, then you can use Enhancer for YouTube’s “Playback speed” feature to have smaller speed steps. Then you can hold ctrl and use the scrollwheel (while over the video) to change the video’s speed by the amount you chose (I use 0.05 speed variation, mostly changing to 1.05x or 1.10x)

For the more advanced, you can also mess around with the userChrome.css file. To create it open a text editor and save the file as userChrome.css in the \Chrome folder in your Firefox profile folder* (make sure to restart Firefox to apply the changes)

To enable the file, enter about:config in the address bar, then accept the warning if it appears. Search for toolkit.legacyUserProfileCustomizations.stylesheets and double-click it to set it to true.

*enter about:support in the address bar, or click on the Help menu > More Troubleshooting Information, then scroll to the Profile Folder line and click on Open Folder. Usually it’s C:\Users\~USERNAME~\AppData\Roaming\Mozilla\Firefox\Profiles\~PROFILENAME~

*Note: the first 2 lines about tab height might not work if you don’t have the Playing/Muted text line in tabs disabled. To disable that line go to about:config, search for browser.tabs.secondaryTextUnsupportedLocales, and add ,en (or the relevant language that you’re using in your system) at the end of the value for it then click Enter and restart Firefox to apply the change.

Here’s the userChrome I use (compiled from different sources)

/*    Tabs/Tab Bar height    */
:root {
 --tab-min-height: 20px !important;
 --tab-max-height: 20px !important;
}

/*    Menu Bar height    */
#toolbar-menubar {
  margin-top: 0px !important;
  margin-bottom: 0px !important;
  padding-top: 0px !important;
  padding-bottom: 0px !important;
  line-height: 22px !important;
  max-height: 22px !important;
}
/* Fixing title bar buttons (close/min/max) due to shortened Menu Bar height */
#toolbar-menubar .titlebar-button {
  padding-block: 1px !important;
}

/* toolbar/address bar/url bar height  */
/* https://github.com/CarterSnich/firefox-xtra-compact/blob/master/chrome/userChrome.css */
toolbar#nav-bar {
	max-height: 30px !important;
}
hbox#urlbar {
	min-height: 22px !important;
}
hbox#urlbar:not([focused="true"]) {
	max-height: 22px !important;
}

/*    Menu Items height/padding    */
menupopup > menu, /* this is the sub-menus/folders/containers */
menupopup > menuitem {
  padding-block: 3px !important; /* above and below each item */
  margin-left: 0px !important; /* margin is the outer space around an item */
  padding-left: 8px !important; /* padding is the inner space inside an item */
  margin-right: 0px !important;
  padding-right: 7px !important; /* otherwise some text in menus gets cut off with ellipses */
}

/* the padding of the menu itself */
menupopup {
  --panel-padding: 1px !important;
}

/* the text part of the item/sub-menu */
menupopup > menu > hbox, /* this is the text part of the sub-menus/folders/containers */
menupopup > menuitem > hbox {
  margin-left: 0px !important;
  margin-right: 8px !important; /* otherwise some text in menus gets cut off with ellipses */
}


menuseparator {
  padding-block: 0px !important;
}

:root{
--arrowpanel-menuitem-padding: 1px 1px !important;
--arrowpanel-menuitem-margin-inline: 1px !important;
--arrowpanel-menuitem-padding-block: 1px !important;
--panel-separator-margin-horizontal: 1px !important;
--panel-subview-body-padding-block: 1px !important;
}
/*(The 2nd number in the first line refers to the left side and right side of the item)*/


/*    Spacing/padding around addons icons in the toolbar/urlbar    */

:root {
  --toolbarbutton-outer-padding: 0px !important;
  --toolbarbutton-inner-padding: 7px !important;
}

.toolbaritem-combined-buttons:not([widget-type='button-and-view']),
.toolbaritem-menu-buttons {
  margin-inline: 0px !important;
  margin-inline-start: 0px !important;
  margin-inline-end: 0px !important;
}

#tabs-newtab-button {
  padding-left: 3px !important;
}

for anybody that wants to disable it, go to the settings and search for “Allow websites to perform privacy-preserving ad measurement”

(or through the dom.private-attribution.submission.enabled flag in about:config)

https://support.mozilla.org/en-US/kb/privacy-preserving-attribution