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;
}

Sorry, it was unclear in my comment. By “it’s actually part of the Mozilla Corporation” I was referring to Firefox, not Mozilla Foundation

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

Just adding that as I understand this, donations to the Mozilla Foundation cannot go towards Firefox, because it’s actually part of the Mozilla Corporation. To help with funding Firefox people can consider purchasing the Corporation’s other products (VPN/Relay/Monitor), or purchasing merch.

See more here on the AMA on Reddit, and this thread

On the humorous side:

• My Brother, My Brother And Me - 3 brothers giving not-so-serious advice to listeners, reacting to questions from Yahoo Answers (when that was a thing), and overall being funny/silly
• My Dad Wrote A Porno - 3 people reacting to an “erotica” book series that one of the guys’ dad published (I would say the books are maybe only 5%-20% explicit, depending on the chapter) (some of the characters’ voices can be annoying/grating, and the narrator tends to repeat sentences after they react to them which can be annoying as well)
• No Such Thing As A Fish - the behind-the-scenes staff of the show QI bring up interesting facts and tidbits from history/nature/etc. (each episode is split into 4 parts where each member brings up a fact and the others react to it and bring up related facts)
• If I Were You - Jake & Amir from CollegeHumor giving advice to listeners (mostly in a sarcastic/tongue-in-cheek way but sometimes genuinely), mostly about relationships/dating
• SmartLess - Jason Bateman and Will Arnett (Arrested Development) and Sean Hayes (Will & Grace) interview a famous person each episode where only one of the hosts known who it is beforehand (it gets better after the first few episodes, though some conversations are less funny/entertaining than others)
• Office Ladies - Jenna and Angela from The Office (US) reacting to each episode of the show and bringing up behind-the-scenes stuff (some of episodes include interviews with other cast members/staff)

Not sure if they all fit entirely but:

• The Story Of Stuff (Annie Leonard)
• How The World Works (Noam Chomsky)
• Predictably Irrational: The Hidden Forces That Shape Our Decisions (Dan Ariely)
• The Hidden Brain (Shankar Vedantam) / Idiot Brain (Dean Burnett)
• The Myth Of Choice (Kent Greenfield) / The Paradox Of Choice (Barry Schwartz)
• The Free Will Delusion: How We Settled For The Illusion Of Morality (James B. Miles)
• Getting Free: Creating An Association Of Democratic Autonomous Neighborhoods (James Herod)
• The Best That Money Can’t Buy (Jacque Fresco)
• No Contest: The Case Against Competition (Alfie Kohn)

Especially for sleeping issues, but also for eye strain, I recommend using a program like flux (for a computer) that allows you to set the color temperature for the day and the gradually lower it further at evening and night. This along with dimming or using different lower temperature lightbulbs in the evening/night (not being exposed to bright/florescent lights) really helped me with falling asleep.

edit: also, lowering the brightness on the monitor itself (mine is set to 27) and in the graphics card control panel (39 for me), and then manually lowering it further in the evening/night in the control panel (25>15>9>0 is how I do it)