I'm not sure if this feature needs to be native, unless Firefox somehow breaks the ability for Grease/Tampermonkey to perform.
tl,dr: with Userscripts I can automate the process of making a frequented website's UI more accessible and less cluttered. I also use it for condition based automation by listening for certain events or watching for JS functions or injecting my own JS.
I have to use a specific website for work to manage a fleet of drivers and dispatch them to service calls, I have to do this from a laptop mounted in my work vehicle because I am also part of the fleet. This website is not designed to be used the way I use it (while driving, shh) so I need to modify the layout so that the functions that I use regularly can be quickly and more easily seen and interacted with so that my eyes are not pulled away from the road for long periods trying to scroll down the page or moving my cursor too far.
Also, the website refuses to remember ANY of your preferences so every time you load the page I have to select dropdowns and resize frames and remove un-needed and oversized elements that take up way too much screen real estate, etc. etc. etc. Any time the website loses connection with the server, which happens a lot because my phone is my hotspot and I drive all over the city constantly changing from cell tower to cell tower and get out of my vehicle with my phone alot, I have to reload the page and redo all of my preferences while trying to drive.