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There are 2 options to fix this problem.
Option 1: Set scrollbars to always display in Windows 11. This is an operating system setting so it will impact all applications.
Go to Settings > Accessibility > Visual effects
Set "Always show scrollbars" to On
Option 2: Change settings in Firefox
This will only impact Firefox but takes more effort. Note Note: there may be other settings in about:config which adjust these scrollbars as well. If you find something new/different, please pass it along.
Open a new tab
Go to about:config
Accept any risks
Search for widget.non-native-theme.scrollbar.style
Change value from 0 (Default OS) to 4 (Windows 10 style)
Search for widget.windows.overlay-scrollbars.enabled
Change value from true to false
Search for widget.non-native-theme.win.scrollbar.use-system-size
Change value from true to false