pocket.enabled to false

First thing I do every clean install

Setting full-screen-api.warning.timeout to 0 removes the notification at the top of the screen that tells you how to exit fullscreen mode.

Plenty.

For me:

browser.ctrlTab.recentlyUsedOrder: False

browser.tabs.insertAfterCurrent: True

dom.popup_maximum: 100

browser.download.improvements_to_download_panel: False

On laptops, I really recommend everyone sets apz.overscroll.enabled to true. It adds the MacOS/iOS overscroll effect when you fling a page fast and you reach the bottom. It does not add any functionnality but it just makes everything so much smoother in my opinion

browser.tabs.tabmanager.enabled -> false
It removes the tab manager. The down facing arrow next to the minimize button at the top right.

extensions.pocket.enabled -> false
Disable Pocket

identity.fxaccounts.enabled -> false
Disable Firefox account

media.autoplay.blocking_policy -> 2
You have to click the play button to play videos.

media.peerconnection.enabled -> false
Disable WebRTC

browser.self_destruct_shortcut -> disabled

gets me everytime

toolkit.legacyUserProfileCustomizations.stylesheets ⇾ True

to load CSS themes that make Firefox, your Firefox

Here a theme's collection https://firefoxcss-store.github.io/

If you're lazy, librewolf is hardened firefox (sometimes it can break sites though)

I like resistFingerprinting = true but it may be a bit much for some people. You lose some keyboard hotkey capabilities in RES and all your windows always open at a certain size upon relaunch/new window.

I have a slow connection, so privacy.partition.network_state = false was a game changer. It prevents firefox to re-download an image (or video, or whatever) when you open it in a new tab (State Partitioning in Mozilla docs).

Warning: the main goal of State Partitioning is to prevent cross-site tracking, so this option will make that easier. In my case it's worth it and I mitigate that by other means, but you should be aware of it.

dom.event.clipboardevents.enabled - block sites from preventing you using copy+paste e.g. in email and password fields.

I've only recently started using this one, so ask me again in a couple of months if it solves the issue :] or if it has unwanted side-effects - I know at least it doesn't prevent websites interacting with the clipboard entirely e.g. with a button to click to copy text to the clipboard

Not obscure but general.smoothScroll.msdPhysics.enabled=true is a must have IMO.

Backspace behavior: 1 something like that. Sets backspace to go to previous page it was once default but it is not anymore

I always like to start over with a clean slate. The one that's been with me for about 15 years now is browser.tabs.closeWindowWithLastTab set to false.