I should have made proposals on the site if I had found how to create an account.
This extension is a good idea (and replaces things that no more run), but needs to be improved.
By the way it appears that critics for two different extensions appear on the same page here.
I tested the one with Web Extension, as I use Firefox Quantum, version 59.0.2 - I presume this extension is somewhat recent.
About the results, I see the active tab displayed with a different colour, and this is why I installed the extension.
That being said, this background colour is at random, and the foreground colour (font colour) is fixed (in the options) so this not always optimum.
I regret not being able to choose colours for other tabs, classically a set of colours for not loaded tabs, another for inactive tabs ...
Tab Mix Plus has accustomed us to use a nice user interface to choose the colours, well let us forget that for the moment being, it takes some time to develop.
What would really be useful is that the extension sets a style name for the active tab, one for the tabs that are no more loaded, one for the other ones. And also that the documentation gives those style names.
With that, we could personalize properly those styles in userChrome.css (at least the advanced users could do that).
In a second time, as an options interface is proposed, it would be nice that for each of these styles it allows to give a foreground colour, and a background colour. Some of them miss. If time allows, some other attributes as bold could be smart too.
When this urgent stuff is done, I should like to mention that after installing the extension I had to style my personal bar (placestoolbar), as the extension modifies its background colour, and the font colour it had was no more adapted.
If you have time to do some tests at this level, it will help, too.
Thanks for the good job.