TL;DR: Great, simple, useful. Install it.
Exactly what I was looking for, and the no-restart install is a strange and wonderful thing that I hope to see more of in other addons.
I how much RAM this thing takes up, but it does have to be enabled forever if you want the color to stay. Even with all of the recent updates, Firefox is still a heavy load, and more addons don't help that.
There's also only 70 colors to choose from. I found the exact color I was looking for, and maybe a million colors would be excessive, but I wouldn't mind an RGB slider.
When switching to Private Browsing Mode, the button would normally switch from orange to purple to add an obvious visual cue to an important change in what information was now being saved to your computer. With this addon (as of version 1.0) the button stays the same color you chose, with only the thin outline of the button switching to purple
The outline and gradient are mostly unwelcome. For most themes, the gradient on your chosen color looks fantastic, but some people like a bleak, monochrome look and while this would help immensely towards that, it still wouldn't be perfect. The outline is a mixed bag of pro and con. while in normal mode, there's a faint orange-red outline to whatever color you choose. Depending on your choice, you might not notice it at all. When in Private Browsing Mode, the outline is replaced with a faint dark-purple outline.
Lastly, I would like more options with this addon like removing the gradient and outline, or choosing a second color for Private Browsing Mode. Adding more options might take away from its "no restart" install, and will likely take away from its simplicity and ease of use.