Just like I predicted in the entry from Jul 02, 2020, removing the
<label> bit me. Chromium-based browsers render
<input>-attached pseudo-elements just fine, but Firefox didn't, which meant that users of Firefox on mobile devices couldn't use the navigation menu. Thankfully, I found out immediately after sharing the address.
It took a while to confirm – I wasn't sure the issue was precisely related to the checkbox that control the navigation menu – and it took its own while to add back the required elements and styles to all the pages.