Firefox favicon .ico retina size bug

Firefox chooses the favicon dynamically from an .ico file based on display density, but it doesn't always seem to choose the most appropriate size.

Steps to reproduce the issue

• Move the Firefox window between a retina screen and a non-retina screen.
• Observe that on the retina screen, Firefox chooses the 96px favicon instead of the 32px, resulting in a difficult to read icon.

Expectation

I expect that the favicon will choose either the 16px or 32px version depending on whether the screen is retina or not. It doesn't seem correct to downscale a drastically larger icon since favicons have very limited real estate, and the smaller designs will typically look better scaled up.

Image reference table

Image	Size
16 × 16
32 × 32
96 × 96

Computer information

• MacBook Pro (16-inch, 2019)
• 3072 × 1920
• This bug happens at all possible resolutions for this built-in laptop display.
• macOS Monterey (12.4) [21F79]

External monitor

• 27-inch
• 2560 × 1440
• 60 Hz
• HDMI

Source code

Source code is available on GitHub.