Interesting... I use Firefox as my primary Android browser for various reasons but I've never seen this. Just checked and still don't see the box icon when I tap the URL bar.

This interested me so I checked the Firefox support site and supposedly it's an option under "settings->display" but I still don't see it. (https://support.mozilla.org/si/kb/scan-qr-codes-firefox-andr...)

Checked the Firefox support site and apparently you need to install a separate app to enable this (https://support.mozilla.org/en-US/kb/scan-qr-codes-firefox-a...)

After installing the app (https://play.google.com/store/apps/details?id=com.google.zxi...) I do indeed have the option and the box.