The active tab background color is barely visible. Please make it darker to stand out against inactive tabs...
Firefox 121.0 Windows 10 x64
It was fine on Firefox 120.0

Not the best, according to my point of view, but thank you for trying - it's interesting.

Very pretty but I have to toggle it each time I start the browser in order for it to animate

Would be almost perfect, except for one glaring issues that had caused me to switch to a different theme. There is insufficient contrast between the active tab and other tabs. If you have many tabs open it is difficult to determine which is the active tab to close it. This causes enough friction in working in Firefox that it became a regular annoyance. For me it is a basic requirement that the active tab should be clearly indicated in the tab bar. This otherwise excellent theme fails in this regard.

bien

edit color -select TAB
please.

Makes the menu bar practically invisible

Good, but really needs contrast on the selected tab background, I can't tell which tab is opened

Looks ok but the animation isn't very wide, so it's repeated 4 times across my screen when maximized.

On Linux, for some reason this theme makes all videos stutter and scrolling extremely sluggish. Once I changed, everything went back to great.

I very much like the theme. My only critique is that I don't like that I cannot see separators on the book marks bar. It just looks like an extra wide space. Fix and I'll change to 5*
EDIT: I don't see a bookmark specific component, perhaps its controlled by the "toolbar_vertical_separator"