User Agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.142 Safari/537.36
Steps to reproduce:
Here is a simple code to reproduce. I kept the wekbit prefix so we can compare with Chrome
HtML: <div class="box">some text</div>
A jsfiddle link: https://jsfiddle.net/wx3v5ct9/
The actual result is a red only background.
We should see a yellow background and the text in red. It works fine on Chrome.
It also works fine if the text layer is at the bottom, but we have to reduce the top layer to see it.
Here is two SO questions where I am using this technique that fail in Firefox: