ClassicTheme ColorModeToggle shows both icons in Firefox

[mattcorner]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

When a Docusaurus 3 site using the classic theme is built, in Firefox, the toggle colour mode button shows both icons rather than the single icon for the current mode.

This works fine in other browsers, and works fine in Firefox when using npm run start.

The issue on occurs when running npm run build && npm run serve

Reproducible demo

https://codesandbox.io/p/sandbox/divine-sky-wcnrpr

Steps to reproduce

1. npx create-docusaurus@latest doc classic --typescript
2. npm run build && npm run serve

Expected behavior

Expect the button to be either:

Actual behavior

Actually seeing this:

Your environment

• Public source code: N/A
• Public site URL: N/A
• Docusaurus version used: 3.0.0
• Environment name and version (e.g. Chrome 89, Node.js 16.4): Firefox 120.0 (64-bit)
• Operating system and version (e.g. Ubuntu 20.04.2 LTS): Seen on MacOS and Windows

Self-service

• I'd be willing to fix this bug myself.

[Josh-Cena]

Duplicate of #9493