Superclean од hadouken

userChrome.css for square tabs + disable fade for inactive window:

/ Proton Tabs Tweaks /

/ Adjust tab corner shape, optionally remove space below tabs /

tabbrowser-tabs {

--user-tab-rounding: 0px;


}

.tab-background {
border-radius: var(--user-tab-rounding) var(--user-tab-rounding) 0px 0px !important; / Connected /
margin-block: 1px 0 !important; / Connected /
}

scrollbutton-up, #scrollbutton-down { / 6/10/2021 /

border-top-width: 1px !important;
border-bottom-width: 0 !important;


}

/ 1/16/2022 Tone down the Fx96 tab border with add-on themes in certain fallback situations /
.tab-background:is([selected], [multiselected]):-moz-lwtheme {
--lwt-tabs-border-color: rgba(0, 0, 0, 0.5) !important;
border-bottom-color: transparent !important;
}
[brighttext="true"] .tab-background:is([selected], [multiselected]):-moz-lwtheme {
--lwt-tabs-border-color: rgba(255, 255, 255, 0.5) !important;
border-bottom-color: transparent !important;
}

/ Container color bar visibility /
.tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
margin: 0px max(calc(var(--user-tab-rounding) - 3px), 0px) !important;
}

/* Make sure tab attention dot isn't too high - 10 Dec 2022 */


.tabbrowser-tab:is([image], [pinned]) > .tab-stack > .tab-content[attention]:not([selected="true"]),
.tabbrowser-tab > .tab-stack > .tab-content[pinned][titlechanged]:not([selected="true"]),

firefox-view-button[attention], .webextension-browser-action[attention="true"] {

background-position-y: bottom 5px !important;
}

toolbar-menubar:-moz-window-inactive, #TabsToolbar:-moz-window-inactive {
opacity: 1 !important;
}

:root {
--tab-min-height: 42px !important;
--tab-block-margin: 0px !important;
}