我的手机（Nexus 5x）和平板电脑（Nexus 9）上安装了Android的Firefox。
I have Firefox for Android installed on both my phone (Nexus 5X) and tablet (Nexus 9).
On my phone, some bookmark icons have a red background:
Whereas on my tablet they appear "normal":
What does the red background mean? I couldn't find an explanation online. Even on Mozilla's help website the YouTube icon has a red background.
It's a small cosmetic feature of Firefox: dominant color as favicon background.
To put it simply, Firefox will try to calculate the dominant color of an icon, and apply it as the background. Here is the screenshot of the feature while it's WIP (might not apply to current version).
Image credit to Margaret Leibovic
As for the reason why some of the icons have it or not, it might be probably a bug on Firefox.
Further reading: Dominant Favicon Color, Revisited on Android - Margaret Leibovic