I feel you, so I have researched this:

For Firefox you can change the width /style of the scrollbar:

(A) In a new tab, type or paste

about:config

in the address bar and press Enter/Return. Click the button accepting the risk.

(B) In the search box in the page, type or paste

widget.non-native-theme.scrollbar.style

(C) Press the Return or Enter key to find the setting. Click the Edit (pencil) button on the right side of the widget.non-native-theme.scrollbar.style setting.

(D) Delete the current 0 value for the default OS style scrollbar. Then input the value 1 (Mac OS X), 2 (GTX), 3 (Android), 4 (Windows 10), or 5 (Windows 11) in the widget.non-native-theme.scrollbar.style box for the scrollbar style you want to change to. For example, enter 4 to change the scrollbar to the default Windows 10 design.

(E) Click the Save button on the right side of the widget.non-native-theme.scrollbar.style setting to apply.

Also for the hiding:

Windows: Settings > Ease of Access > Display > Automatically hide scroll bars in Windows

Mac: System Preferences > General > Show scroll bars