I'm a little disappointed. Not bad but has to be improved.
For Alex Sirota :
I used for many time a very simple tool (colorPicker by Ryan Tate), but this extension is out of date because not compatible with Firefox 57+. By a small choice, I tried again ColorZilla. He's too heavy that I need (for this reason I prefer colorPicker), but I like some options, like paste HEX code in lowercase and the webpage color analyzer that could be useful.
But, the cursor seems not to be very accurate and the main problem is that the menu of ColorZilla is too width and too slow to disappear! As many users, I work on a web design and I want to pick color faster than the speed of light ;) I'm sure your worked hard on this extension, but if the WebExtension API can do, just open the menu by a right click (or with a special menu button) and keep the left click just to start the color picker with the button and to get color, could be a good way in my opinion. And after the color is in the clipboard, do not wait to close the tool bar. Please do it, and I give you 4 or 5 stars, I'm sure like many users ;)