They don’t support Linux applications, they don’t support the main Linux browser. They don’t want you to use Linux

117 deprecate appearance: -moz-win-borderless-glass what is the alternative now ?

I have this css theme https://github.com/Neikon/Fox11 and this is my config if some one is interested

adaptive-tab-bar-colour

the permissions control is done by firefox, and as you can see in the add-ons settings it only asks for access to this list of websites

Access to your data from sites in the domain googlevideo.com
Access your data from sites in the domain youtube.com
Access your data from sponsor.ajay.app
Access your data for dearrow-thumb.ajay.app
Access your data from www.youtube-nocookie.com

and optionally the user can give you access to all sites but it is not necessary. This can be changed but in that case firefox will warn you that the plugin requires new permissions.

toolkit.legacyUserProfileCustomizations.stylesheets ⇾ True

to load CSS themes that make Firefox, your Firefox

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