userChrome.css

/* UserChrome.css for Firefox */

/* Remove maximum/minimum width restriction of sidebar */
#sidebar {
    max-width: none !important;
    min-width: 0px !important;
}


/* Inactive tabs: Outline style */

.tabbrowser-tab:not([selected]):not([multiselected]) .tab-background {
    border: 1px solid rgba(0, 0, 0, .20) !important;
    border-bottom-color: transparent !important;
}

/* For dark backgrounds */
[brighttext="true"] .tabbrowser-tab:not([selected]):not([multiselected]) .tab-background {
    border: 1px solid rgba(255, 255, 255, .20) !important; /* Simplified color setting */
    border-bottom-color: transparent !important;
}


/* Emulate blue tab line from Photon 57-88 UI */
.tab-background::before {
    content: "";
    display: block;
    height: 2px;
    margin: -1px var(--user-tab-rounding) 0;
}

.tabbrowser-tab[selected] .tab-background::before {
    background-color: #0a84ff;
}

.tabbrowser-tab[multiselected]:not([selected]) .tab-background::before {
    background-color: color-mix(in srgb, #0a84ff 66%, transparent);
}

.tabbrowser-tab:not([selected], [multiselected]):hover .tab-background::before {
    background-color: color-mix(in srgb, currentColor 33%, transparent);
}


/* Unload tabs line from Solarize Fox Theme Instructions*/
.tabbrowser-tab[pending] .tab-background::before {
    background-color: red;
}


/* Override font-size for tabs */
.tabbrowser-tab {
    font-size: 13px !important;
}

/* Remove extra padding between tabs */
.tabbrowser-tab:not(:first-of-type) {
    padding-left: 0 !important;
}


/* Proton Tabs Tweaks */
#tabbrowser-tabs {
    --user-tab-rounding: 5px;
}

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


/* (likely obsolete) */
/* Adjust scroll buttons */
/*
#scrollbutton-up, #scrollbutton-down {
    border-top-width: 1px !important;
    border-bottom-width: 0 !important;
}
*/


/* Potentially obsolete */
/* 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;
}
*/