Restore native scrollbar support?

### Is your feature request related to a problem?

Firefox no longer supports native scrollbars, they have their own custom scrollbars designed to look like the Windows 10 ones. This is a problem if you have anything other than the default Windows 10 theme, such as Windows 7 aero, some sort of classic theme, or any custom theme that changes the scrollbars.

### The solution I'd like

Since this is "firefox UI fix" is it possible to fix this issue? It's been suggested before on Reddit and I just wanted to make it more official by suggesting it here.

### Screenshots or video

_No response_

### Alternatives I've considered

Using an older version of the browser that still supports native scrollbars, moving to a different browser.

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by investigating how Firefox UI Fix currently styles scrollbars and whether native scrollbar behavior can be restored for non-Windows 10 themes; done should mean a documented, working solution that preserves the requested custom-theme appearance.

Written by the indexing model from the issue text.