Feature request: Add option to bring back original circle-shaped PiP button

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

The original `Picture-in-Picture` button shape was a circle, but it was replaced by a rounded square.

### The solution I'd like

Add an `about:config` option to restore the original circle-shaped PiP button.

### Screenshots or video

Original button shape:

Actual button shape:

### Alternatives I've considered

_No response_

### Additional context

- https://bugzilla.mozilla.org/show_bug.cgi?id=1811312

Start by locating the existing Picture-in-Picture button styles and any about:config preference handling in the repository; the issue names no files or tests. Use Bugzilla 1811312 for context, then verify that the new option restores the circle shape without changing the default appearance.

Written by the indexing model from the issue text.