You can get the thinner Windows 11 style scrollbars by going to about:config in the address bar and changing widget.non-native-theme.scrollbar.style to 5. Make them appear on hover by changing widget.non-native-theme.win11.scrollbar.force-overlay-style to true. Restart the browser to apply changes.

WARNING from the moderator team: Changing Firefox preferences from the about:config page can sometimes break Firefox or cause strange behavior. You should only do this if you know what you're doing. Please read Firefox Advanced Customization and Configuration Options to learn more.

the second method didnt work on win10

Since those didn't work maybe those are Windows standard settings and you should also post that question to a Windows forum to find more solutions as well.

Go to about:config and create ui.useOverlayScrollbars (Number) with value 1.

WARNING from the moderator team: Changing Firefox preferences from the about:config page can sometimes break Firefox or cause strange behavior. You should only do this if you know what you're doing. Please read Firefox Advanced Customization and Configuration Options to learn more.