How to change color format in Firefox Dev Tools?

For those coming across this later, like I did: it has now shipped. You can change colour by holding Shift and clicking the colour sample (i.e. the circle filled with the colour).

You can't do that at the moment in Firefox DevTools, but this is being worked on as we speak. You can follow along the development here: https://bugzilla.mozilla.org/show_bug.cgi?id=1136257 I expect this to land for Firefox 40.