I had to search several files in the trashcan and make some tests, but I got it.

The alert in Firefox is:

Error in parsing value for 'display'. Declaration dropped.

And it was caused by a typo ("break" instead of "block"):

sample.style.display = "inline-break"

That's what I most hate in web development: silent errors (are you listening, Google?).

So, no relation to the color picker API.

As I am not comfortable with native color pickers (too much variation, radical change at any update), I didn't think twice (I "guessed" the value for 'display' was about the color to be displayed) before deciding to adopt my homemade solution. Also I barely use Firefox.