I'm slowly thinking about updating it. After half a year has passed…

cheers
also, i urge anyone wanting to do this do read the comments in the .css files
(css comment example: /* comment, can be span in multiple lines of code */)
btw you need to restart firefox after changing anything in the .css files if that helps

open about:config
paste this in toolkit.legacyUserProfileCustomizations.stylesheets and set it to true
open about:support and open profile directory
inside the directory create a folder named chrome (the 'chrome' isn't owned by google and is actually an browser element)
go inside the chrome folder
in it copy the userChrome.css and userContent.css files, with their respective contents (you can also name the files however you like and use the @inport rule from CSS
obtain images of sacred knowledge that's even the deepest corners of 4Chan haven't seen and name it firefox.png
restart firefox
be happy
contemplate life choices
think about the meaning of life
get up and do something life-changing, like starting a project. Doesn't have to be big or something. Even the smallest things cound

in this example I use those by lux
https://twitter.com/LuxxArt/status/1727019006186664230
folder example:

better examples coming in the future

this one on almost every firefox built-in page:

(sidenote: to change the padlock icon to custom thingy you need to uncomment two lines of code in the userChrome.css
why?, because i'm not sure if there are more padlock icons. for now the commented configuration is set to change for when tooltip is either:
"Verified by" or "" [on firefox about:*])

about:preferences:
i've tried a lot

and probably other things i forgot about

i'm going to set those on my school's PCs
update half a year later: I did it they can count on that

you can message me on my dc if you find something new/undone (there is): 42majorspellingmistakes
I'm better known as Lumi Tassu


good luck and have fun with your firefox o7