Dynamic theme for Firefox that uses the current website's "primary" color
A dynamic Firefox theme that changes the theme to the current website's "primary" color. Available in Firefox Add-ons.
Order of precedence for determining the color:
Color Tailor was originally created for the Firefox Quantum Extensions Challenge. I wrote a blog post detailing my experience.
Install Color Tailor from the Add-ons page. Firefox 63 and above is required.
Color Tailor was inspired by an iTunes feature that would change UI colors on the fly to match album covers.
Chrome on Android does a similar thing with the toolbar color, and there is an open issue to do the same for Firefox.
Vivaldi has this feature built-in.