These shortcuts work when the breadcrumbs bar is focused.

These shortcuts work when you’re in the Inspector’s CSS panel

Keyboard shortcuts for the Firefox JavaScript Debugger.

Note: Before Firefox 66, the combination Ctrl + Shift + S on Windows and Linux or Cmd + Opt + S on macOS would open/close the Debugger. From Firefox 66 and later, this is no longer the case.

Source: mozilla.org