As someone who loves to decorate herself and her things with fun and bright colors and things. I've always decorated Firefox with it's many themes but that was starting to get old. I wanted something DIFFERENT! So, I found this and after going through many other variations, I am happy to say I've found the one! I LOVE being able to have a rainbow of tabs on my screen, especially when the colors are MY choice! It's easy and quick and the options are limitless!!1

Ignore the "no GUI" comment, don't know what happened there.
To add a new domain, simply navigate to a page on that domain, click on the icon in the toolbar (or add-on menu) and a simple sidebar opens to select a desired color for the domain.
There's also a Settings button that leads to the Options view in add-ons manager where you can delete (but sadly not change) saved colors.

I'm giving it 5 stars, because it is simple to configure, I have had no problems despite the last update being 2 years ago, and it does what I need it to do.

One problem is tho that when you have multiple windows open, and one of them has a colored domain open, it will color everything. If the windows both have a (different) colored domain open, then it follows the focus.
Although the name says Tabs, the description clearly states that it will set the browser window a custom color, so that was expected

Cleverly thought! I always dread doing something stupid on the production websites, this protects me from myself, thank you Joe! :-)

Smart Add-On, Thank You! :)

UPDATE 2.Sept.2020 ERROR: Cannot save the Color with FF 80.01 on Windows 7 = Push the ''save'' Button jump it back to White. Works flawlessly on FF 68.12 ESR.

The update works great, thanks!