Firemonkey is a totally new combined user-Scripts and user-Style manager. While it has similar functions to other user-Script managers like GreaseMonkey/Tampermonkey/Violentmonkey, and user-CSS managers like Stylish/Stylus/xStyle, there are also differences. Note: None of my extensions contain any tracking, advertising or privacy infringement. They all have Mozilla Open Source licence. The source is clear and accessible by extracting the XPI. Note: Firefox for Android support is experimental (v2.12+) Features

Lightweight extension for both User-Scripts and User-Styles
Secure dedicated API (Firefox 65+) to handle User-Scripts
Turn Scripts/CSS On/Off from Toolbar Button
Supports both GM3 & GM4 (GM_*** and GM.***) style functions plus some new API (e.g. fetch)
Script/CSS Auto & Manual Update
Export/Import Preferences (for backup or share) to/from a local file on your computer
Export/Import Scripts & CSS
@require for requiring other saved scripts
@container support
Web Install scripts from GreasyFork, OpenUserJS & Stylish userstyles
Direct Install scripts loaded into tab from GreasyFork, OpenUserJS & file:///
Console logged script errors are referenced to the individual script
Temporarily insert existing userScript & userCSS
Scratchpad to temporarily insert CSS & JavaScript
Log to display latest error & script update messages
Notifications from scripts show script's name
Help Guide included

Permissions

Input data to the clipboard: Used to Copy to Clipboard
Download files and read and modify the browser’s download history: Used only to export Script/CSS/Preferences
Display notifications to you: Used to inform users
Access browser tabs: Used to get data for tab operations
Store unlimited amount of client-side data: Used by the user to store Scripts & CSS
Access browser activity during navigation: Used to display the running scripts in active tab
Access your data for all websites: Used to inject Script/CSS

External Reviews FireMonkey uses Firefox's official API for userscripts and userstyles How to use Userscripts, Userstyles with FireMonkey in Firefox?