[Classic Editor] Distraction free writing mode icon not visible on Firefox browser

### Quick summary

When using the Firefox web browser with the "Enable full-height editor and distraction-free functionality" option activated through the "Screen options," the icon does not appear in the toolbar under the "Visual" tab while editing a post in the classic editor. This is different from how it is displayed in Google Chrome.

**Chrome**


**Firefox**

### Steps to reproduce

1. Edit a post in the classic editor using Firefox.
2. Enable the `Enable full-height editor and distraction-free functionality` option in the `Screen Options`.
3. The icon to enter distraction-free writing mode will not appear.

### What you expected to happen

For the distraction-free writing mode icon to appear.

### What actually happened

The distraction-free writing mode icon does not appear.

### Impact

Some (< 50%)

### Available workarounds?

Yes, easy to implement

### If the above answer is "Yes...", outline the workaround.

Use another web browser such as Google Chrome

### Platform (Simple and/or Atomic)

Atomic, Simple

### Logs or notes

_No response_

Start by reproducing the issue in the classic editor with Firefox, the Visual tab, and the Screen Options setting enabled, then compare the toolbar behavior with Chrome. Locate the classic editor toolbar entry point and verify that the distraction-free writing mode icon appears in Firefox as well as Chrome.

Written by the indexing model from the issue text.