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.
It pops up whenever a KDE application offers colour selection. Easiest way is probably to open settings to Appearance > Colors and click the Custom button.
Falkon also uses it with input type=color which is why the screenshot says Falkon in it.
IMHO a better design would be a mix between the two.
The left one has much better color palettes. Bigger, with a much nicer color selection than just "neon colors" on the right one. But without a color selector/slider it's unusable for advanced users. Should have an expandable panel with the hue sliders. And a color picker if I want to take it from somewhere else in the screen
Heaven forbid someone put hue on x axis and saturation on y axis and have a separate slider for value and allow you to manually input and allows you to save your favourites in a color picker they made.