Detect favicon average luminance and adapt icon background for contrast on tabs

[miles]

Favicons allow transparent pixels, which means they interact directly with the color of a tab (as the theme in use would dictate). The browsing experience could be improved if Firefox detected low contrast combinations--for example, a site favicon that is a black or dark logo with a transparent background, designed with the expectation of a light tab behind it. (https://seatylock.com/ is a current example when using a dark theme). If Firefox could detect that a favicons pixels are on average below a certain threshold, it could then apply a light background to boost the icon's contrast, or alternatively invert the design. Some websites (example https://www.nytimes.com/) in fact detect dark theme preference and then adapt the favicon dynamically. This feature would enhance visibility and accessiblity for sites that don't offer that.

[Jon]

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