Hex Preview от Lucy Havok

A lightweight browser extension that - upon selection of a valid hex code - displays a swatch of the colour it represents. By default, it also displays the RGB values represented in decimal, and the Alpha value represented as a float between 0 and 1 (like CSS). Alpha value display format can be changed to its decimal representation in the add-on's options.

Supported formats:
#RRGGBB
#RRGGBBAA
#RGB
#RGBA
RRGGBB
RRGGBBAA
RGB
RGBA