Excuse me for not answering the question but are you using the color picker in Firefox really that frequently?
I would just choose a nice color with the picker of my choice and paste the hex-code / type the RGB in the custom panel of the default FF color picker and that's it.
I'm not a dev, but I'm wondering if the 'Plasma Browser Integration' package has any influence on the picker? Or maybe there are GTK app settings in play.
Just a thought.
It really annoys me that Firefox is not more integrated in KDE than it is.
I have the plasma browser integration. It adds an item to “Share” in the context menu (for use with KDEconnect etc) and integrates Firefox with the media player.