[Bug]: Scrollable content in dialogs are focusable on Firefox

### Code of conduct

- [x] I agree to follow this project's code of conduct.

### Impacted component(s)

sp-dialog

### Library version

1.7.0

### Expected behavior

Scrollable regions should not be focusable when there is already focusable content within the region.

### Actual behavior

Scrollable regions themselves are focusable on Firefox.

Context: https://bugzilla.mozilla.org/show_bug.cgi?id=1069739

### Screenshots

### What browsers are you seeing the problem in?

Firefox

### How can we reproduce this issue?

1. Go to https://stackblitz.com/edit/vitejs-vite-mki18jny?file=src%2Fmy-element.ts,package.json
2. Click on Open dialog button
3. Tab through the dialog and notice the div being focusable

### Sample code or abstract reproduction which illustrates the problem

_No response_

### Severity

SEV 3

### Logs taken while reproducing problem

_No response_

### Would you like to track this issue in Jira?

- [x] Yes, please tell me the ticket number!

Start with the sp-dialog component and reproduce the issue using the linked StackBlitz example in Firefox. Tab through the open dialog and compare its focus behavior with the expected behavior: scrollable regions should not receive focus when they contain focusable content.

Written by the indexing model from the issue text.