Sadly, this add-on no longer delivers what it advertises, it only acts as a donation trap.
I used it with the previous versions of Firefox, and really sad to see it go - but it is gone, have no doubt, as other add-ons could already solve this problem with the new Firefox versions. Just go with Tree Style Tabs and TST Colored Tabs.

This add-on would've been great if it was compatible with Tree Style Tabs; which it isn't and that's okay.

I wouldn't be surprised if all ~50k users of this add-on does NOT use Tree Style Tabs, but I'm surprised this isn't a recommended add-on considering the amount of users.

Was a once great extension. Now I'm seeing really annoying behaviors. For one, the color of the tabs constantly change in Windows. In Linux it causes it to rapidly jump from one tab to another. I understand that this may not be entirely the developers fault. Part of it is the buggy framework Mozilla has set up for extensions, which is unfortunate. What separated Firefox from other browsers used to be my ability to customize Firefox to work the way I want to. As it stands now, other once great extensions no longer work correctly or are severely crippled. Not sure why I should continue to use Firefox over Chrome

I was really not happy to lost my session and got your "donation" page instead of it.

If tab bar move to bottom screen (use userChrome.css) and
@-moz-document url("chrome://browser/content/browser.xhtml") {
:root:not([inFullscreen]) #browser { margin-bottom: 28px; }
:root:not([inFullscreen]) #TabsToolbar { position: fixed; display: flex; bottom: 0px; width: 100%; }
#tabbrowser-tabs { flex-grow: 1; }
#tabbrowser-tabs > .tabbrowser-arrowscrollbox { width: -moz-available }
#TabsToolbar-customization-target { width: 100%; }
.toolbar-items { width: 100%; }
}
addons dont work.

I would gladly change mu one star to 5 stars once dev. changes it to work as it used to work. I want to change the tabs and nothing else, I also want different coloring on different tabs, like one for active tab, one for the others, per site coloring.
But I don't want my entire browser to change color. :(
I really hope this previously great extension can be fixed.