Add-In&Out - the app is working INside the Firefox profile in use and OUTside the Firefox running time: everything  is done before the Firefox start -

Safe - only one app to add many styles and new unique features without touching your personal data: history, bookmarks, addons, permissions and settings. Your "chrome" folder (if any existed) will be saved in the Backups > "FirstRun" folder during the setup - whenever you want you can restore your pre-FirstRun "chrome" folder in two mouse clicks -

Standalone - all the images and .css rules are embedded locally - customize your browsing experience without any waste of internet bandwidth, simply no needs to silently connect to any data hosting or repository and yes you can work offline keeping your full customization -

Easy Setup - you can easily point to the proper profile in use thanks to the new installing procedure with 3 options: the very simple Quick_Setup \ the visual Manual_Setup \ the very precise Guided_Setup -

(Firefox Browser 70 and later ) - Bypass the manual "about:config" procedure to enable and load the chrome folder resources. If needed, during the setup the app will automatically set to "true" the new UserPreference: "toolkit.legacyUserProfileCustomizations.stylesheets"

(Firefox Browser 89 and later) - Bypass the manual "about:config" procedure to continue to show and use the "compact" option in the Menu > More tools > Customize Toolbar > Density. Automatically set to true: "browser.compactmode.show"