Mine It's Firefox Nightly with a highly nightly based interface :) I made the theme myself, it's called Nightly Neon. It can be improved, but I am happy with the result it had in my screen. I am pretty new to this editing stuff, but I could find some nice tweaks around /r/firefox. Here is my userChrome.css:

`@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/===================Hide forward button when not used===================/

forward-button[disabled="true"] { display: none; }

/===================Hide tab close buttons===================/

.close-icon{ transform:scale(1,1)!important; transition:opacity 250ms var(--animation-easing-function), margin-left 250ms var(--animation-easing-function), transform var(--animation-easing-function), visibility 0ms 25ms!important; } .tabbrowser-tab:not(:hover) .close-icon{ opacity:0!important; margin-left:calc((-20px * .9) + 2px)!important; transform:scale(.9,.9)!important; }

/===================Hide bookmark icons===================/

.bookmark-item > .toolbarbutton-icon { display: none !important; }

/===================Cosmetic changes in bars===================/

.tabbrowser-tab[usercontextid] .tab-line{ background-color:var(--identity-tab-color)!important; }

.tab-background[selected="true"] { background-attachment: none!important; background-color: rgba(0,0,0,0.3) !important; background-image: none!important; }

TabsToolbar .tabbrowser-tab[selected] {

color: white !important;

}

.tab-line[selected]{ background-color: rgb(0,255,255) !important; height: 2px !important; }

nav-bar{border-top: 0px !important; background:none !important;}

PersonalToolbar{background:none !important;}`

And my userContent.css (home and new tab background):

`@-moz-document url-prefix(about:home) { #launcher { display: none !important; }

#searchIconAndTextContainer {
    height: 30px;
}

#searchIcon {
    width: 20px;
    height: 20px;

    top: 50%;
    transform: translateY(-50%);
    margin-left: 2%;
}

#searchText {
    font-size: 90%;
    border-radius: 4px 0px 0px 4px;
}

body {
    background: linear-gradient(46deg, #D100F9 0%, #00EF78 100%);
}

#searchSubmit {
    border-radius: 0px 4px 4px 0px;
}

}

@-moz-document url-prefix(about:newtab) { body { background: linear-gradient(46deg, #D100F9 0%, #00EF78 100%) !important; }

.section-title {
    color: rgb(32, 32, 32) !important;
}

.search-wrapper {
    height: 30px !important;
}

}`

Hope you like it :) comments and remarks are welcome!