I wanted a simple addon that lets me color-code tabs so i can easily distinguish between them. E.g. I have 5 tabs opened with 5 different issues in Azure Devops and want one i'm working on to be red and onother to be blue so when I switch beween applications and many other tabs I will always easily wind that one important tab. I don't want the whole browser tab bar to change color when i select the tab, I just want the small tab to be colored, without having to select it. This addon works but is completly useless for me.

It would be amazing if the app had the option to not only change the colour based on domain alone but also include specific url paths.

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

While not as easy as the old XUL Colorfultabs is is better than the web extension of Colorfultabs. That said I wish it could color each tab without the need to select the tab. That would be worth 5 stars and probably more. Good job thus far. However today I found a bug. If I have already saved a color for a domain I can't change it and resave it. I have to delete the color for the domain first and then save it. Definitely not Intuitive.

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!